Re: [PATCH 2/3] KVM: x86: introduce KVM_MEM_PCI_HOLE memory

From: Vitaly Kuznetsov
Date: Thu Aug 06 2020 - 05:09:16 EST

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In reply to: Andrew Jones: "Re: [PATCH 2/3] KVM: x86: introduce KVM_MEM_PCI_HOLE memory"
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Andrew Jones <drjones@xxxxxxxxxx> writes:

> On Tue, Jul 28, 2020 at 04:37:40PM +0200, Vitaly Kuznetsov wrote:
>> PCIe config space can (depending on the configuration) be quite big but
>> usually is sparsely populated. Guest may scan it by accessing individual
>> device's page which, when device is missing, is supposed to have 'pci
>> hole' semantics: reads return '0xff' and writes get discarded. Compared
>> to the already existing KVM_MEM_READONLY, VMM doesn't need to allocate
>> real memory and stuff it with '0xff'.
>>
>> Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> ---
>> Documentation/virt/kvm/api.rst | 19 +++++++++++-----
>> arch/x86/include/uapi/asm/kvm.h | 1 +
>> arch/x86/kvm/mmu/mmu.c | 5 ++++-
>> arch/x86/kvm/mmu/paging_tmpl.h | 3 +++
>> arch/x86/kvm/x86.c | 10 ++++++---
>> include/linux/kvm_host.h | 7 +++++-
>> include/uapi/linux/kvm.h | 3 ++-
>> virt/kvm/kvm_main.c | 39 +++++++++++++++++++++++++++------
>> 8 files changed, 68 insertions(+), 19 deletions(-)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 644e5326aa50..fbbf533a331b 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -1241,6 +1241,7 @@ yet and must be cleared on entry.
>> /* for kvm_memory_region::flags */
>> #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
>> #define KVM_MEM_READONLY (1UL << 1)
>> + #define KVM_MEM_PCI_HOLE (1UL << 2)
>>
>> This ioctl allows the user to create, modify or delete a guest physical
>> memory slot. Bits 0-15 of "slot" specify the slot id and this value
>> @@ -1268,12 +1269,18 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
>> be identical. This allows large pages in the guest to be backed by large
>> pages in the host.
>>
>> -The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and
>> -KVM_MEM_READONLY. The former can be set to instruct KVM to keep track of
>> -writes to memory within the slot. See KVM_GET_DIRTY_LOG ioctl to know how to
>> -use it. The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,
>> -to make a new slot read-only. In this case, writes to this memory will be
>> -posted to userspace as KVM_EXIT_MMIO exits.
>> +The flags field supports the following flags: KVM_MEM_LOG_DIRTY_PAGES,
>> +KVM_MEM_READONLY, KVM_MEM_READONLY:
>
> The second KVM_MEM_READONLY should be KVM_MEM_PCI_HOLE. Or just drop the
> list here, as they're listed below anyway
>
>> +- KVM_MEM_LOG_DIRTY_PAGES can be set to instruct KVM to keep track of writes to
>> + memory within the slot. See KVM_GET_DIRTY_LOG ioctl to know how to use it.
>> +- KVM_MEM_READONLY can be set, if KVM_CAP_READONLY_MEM capability allows it,
>> + to make a new slot read-only. In this case, writes to this memory will be
>> + posted to userspace as KVM_EXIT_MMIO exits.
>> +- KVM_MEM_PCI_HOLE can be set, if KVM_CAP_PCI_HOLE_MEM capability allows it,
>> + to create a new virtual read-only slot which will always return '0xff' when
>> + guest reads from it. 'userspace_addr' has to be set to NULL. This flag is
>> + mutually exclusive with KVM_MEM_LOG_DIRTY_PAGES/KVM_MEM_READONLY. All writes
>> + to this memory will be posted to userspace as KVM_EXIT_MMIO exits.
>
> I see 2/3's of this text is copy+pasted from above, but how about this
>
> - KVM_MEM_LOG_DIRTY_PAGES: log writes. Use KVM_GET_DIRTY_LOG to retreive
> the log.
> - KVM_MEM_READONLY: exit to userspace with KVM_EXIT_MMIO on writes. Only
> available when KVM_CAP_READONLY_MEM is present.
> - KVM_MEM_PCI_HOLE: always return 0xff on reads, exit to userspace with
> KVM_EXIT_MMIO on writes. Only available when KVM_CAP_PCI_HOLE_MEM is
> present. When setting the memory region 'userspace_addr' must be NULL.
> This flag is mutually exclusive with KVM_MEM_LOG_DIRTY_PAGES and with
> KVM_MEM_READONLY.

Sound better, thanks! Will add in v2.

>
>>
>> When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of
>> the memory region are automatically reflected into the guest. For example, an
>> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
>> index 17c5a038f42d..cf80a26d74f5 100644
>> --- a/arch/x86/include/uapi/asm/kvm.h
>> +++ b/arch/x86/include/uapi/asm/kvm.h
>> @@ -48,6 +48,7 @@
>> #define __KVM_HAVE_XSAVE
>> #define __KVM_HAVE_XCRS
>> #define __KVM_HAVE_READONLY_MEM
>> +#define __KVM_HAVE_PCI_HOLE_MEM
>>
>> /* Architectural interrupt line count. */
>> #define KVM_NR_INTERRUPTS 256
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index 8597e8102636..c2e3a1deafdd 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -3253,7 +3253,7 @@ static int kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, gfn_t gfn,
>> return PG_LEVEL_4K;
>>
>> slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, true);
>> - if (!slot)
>> + if (!slot || (slot->flags & KVM_MEM_PCI_HOLE))
>> return PG_LEVEL_4K;
>>
>> max_level = min(max_level, max_page_level);
>> @@ -4104,6 +4104,9 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
>>
>> slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
>>
>> + if (!write && slot && (slot->flags & KVM_MEM_PCI_HOLE))
>> + return RET_PF_EMULATE;
>> +
>> if (try_async_pf(vcpu, slot, prefault, gfn, gpa, &pfn, write,
>> &map_writable))
>> return RET_PF_RETRY;
>> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
>> index 5c6a895f67c3..27abd69e69f6 100644
>> --- a/arch/x86/kvm/mmu/paging_tmpl.h
>> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
>> @@ -836,6 +836,9 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t addr, u32 error_code,
>>
>> slot = kvm_vcpu_gfn_to_memslot(vcpu, walker.gfn);
>>
>> + if (!write_fault && slot && (slot->flags & KVM_MEM_PCI_HOLE))
>> + return RET_PF_EMULATE;
>> +
>> if (try_async_pf(vcpu, slot, prefault, walker.gfn, addr, &pfn,
>> write_fault, &map_writable))
>> return RET_PF_RETRY;
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 95ef62922869..dc312b8bfa05 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3515,6 +3515,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>> case KVM_CAP_EXCEPTION_PAYLOAD:
>> case KVM_CAP_SET_GUEST_DEBUG:
>> case KVM_CAP_LAST_CPU:
>> + case KVM_CAP_PCI_HOLE_MEM:
>> r = 1;
>> break;
>> case KVM_CAP_SYNC_REGS:
>> @@ -10115,9 +10116,11 @@ static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
>> ugfn = slot->userspace_addr >> PAGE_SHIFT;
>> /*
>> * If the gfn and userspace address are not aligned wrt each
>> - * other, disable large page support for this slot.
>> + * other, disable large page support for this slot. Also,
>> + * disable large page support for KVM_MEM_PCI_HOLE slots.
>> */
>> - if ((slot->base_gfn ^ ugfn) & (KVM_PAGES_PER_HPAGE(level) - 1)) {
>> + if (slot->flags & KVM_MEM_PCI_HOLE || ((slot->base_gfn ^ ugfn) &
>
> Please add () around the first expression
>

Ack

>> + (KVM_PAGES_PER_HPAGE(level) - 1))) {
>> unsigned long j;
>>
>> for (j = 0; j < lpages; ++j)
>> @@ -10179,7 +10182,8 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
>> * Nothing to do for RO slots or CREATE/MOVE/DELETE of a slot.
>> * See comments below.
>> */
>> - if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY))
>> + if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY) ||
>> + (new->flags & KVM_MEM_PCI_HOLE))
>
> How about
>
> if ((change != KVM_MR_FLAGS_ONLY) ||
> (new->flags & (KVM_MEM_READONLY|KVM_MEM_PCI_HOLE)))
>

Ack

>> return;
>>
>> /*
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index 989afcbe642f..63c2d93ef172 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -1081,7 +1081,12 @@ __gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
>> static inline unsigned long
>> __gfn_to_hva_memslot(struct kvm_memory_slot *slot, gfn_t gfn)
>> {
>> - return slot->userspace_addr + (gfn - slot->base_gfn) * PAGE_SIZE;
>> + if (likely(!(slot->flags & KVM_MEM_PCI_HOLE))) {
>> + return slot->userspace_addr +
>> + (gfn - slot->base_gfn) * PAGE_SIZE;
>> + } else {
>> + BUG();
>
> Debug code you forgot to remove? I see below you've modified
> __gfn_to_hva_many() to return KVM_HVA_ERR_BAD already when
> given a PCI hole slot. I think that's the only check we should add.

No, this was intentional. We have at least two users of
__gfn_to_hva_memslot() today and in case we ever reach here with a
KVM_MEM_PCI_HOLE slot we're doomed anyway but it would be much easier to
debug the immediate BUG() than an invalid pointer access some time
later.

Anyway, I don't really feel strong and I'm fine with dropping the
check. Alternatively, I can suggest we add

BUG_ON(!slot->userspace_addr);

to the beginning of __gfn_to_hva_memslot() intead.

>
>> + }
>> }
>>
>> static inline int memslot_id(struct kvm *kvm, gfn_t gfn)
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 2c73dcfb3dbb..59d631cbb71d 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -109,6 +109,7 @@ struct kvm_userspace_memory_region {
>> */
>> #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
>> #define KVM_MEM_READONLY (1UL << 1)
>> +#define KVM_MEM_PCI_HOLE (1UL << 2)
>>
>> /* for KVM_IRQ_LINE */
>> struct kvm_irq_level {
>> @@ -1034,7 +1035,7 @@ struct kvm_ppc_resize_hpt {
>> #define KVM_CAP_ASYNC_PF_INT 183
>> #define KVM_CAP_LAST_CPU 184
>> #define KVM_CAP_SMALLER_MAXPHYADDR 185
>> -
>> +#define KVM_CAP_PCI_HOLE_MEM 186
>>
>> #ifdef KVM_CAP_IRQ_ROUTING
>>
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 2c2c0254c2d8..3f69ae711021 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -1107,6 +1107,10 @@ static int check_memory_region_flags(const struct kvm_userspace_memory_region *m
>> valid_flags |= KVM_MEM_READONLY;
>> #endif
>>
>> +#ifdef __KVM_HAVE_PCI_HOLE_MEM
>> + valid_flags |= KVM_MEM_PCI_HOLE;
>> +#endif
>> +
>> if (mem->flags & ~valid_flags)
>> return -EINVAL;
>>
>> @@ -1284,11 +1288,26 @@ int __kvm_set_memory_region(struct kvm *kvm,
>> return -EINVAL;
>> if (mem->guest_phys_addr & (PAGE_SIZE - 1))
>> return -EINVAL;
>> - /* We can read the guest memory with __xxx_user() later on. */
>> - if ((mem->userspace_addr & (PAGE_SIZE - 1)) ||
>> - !access_ok((void __user *)(unsigned long)mem->userspace_addr,
>> - mem->memory_size))
>> +
>> + /*
>> + * KVM_MEM_PCI_HOLE is mutually exclusive with KVM_MEM_READONLY/
>> + * KVM_MEM_LOG_DIRTY_PAGES.
>> + */
>> + if ((mem->flags & KVM_MEM_PCI_HOLE) &&
>> + (mem->flags & (KVM_MEM_READONLY | KVM_MEM_LOG_DIRTY_PAGES)))
>> return -EINVAL;
>> +
>> + if (!(mem->flags & KVM_MEM_PCI_HOLE)) {
>> + /* We can read the guest memory with __xxx_user() later on. */
>> + if ((mem->userspace_addr & (PAGE_SIZE - 1)) ||
>> + !access_ok((void __user *)(unsigned long)mem->userspace_addr,
>> + mem->memory_size))
>> + return -EINVAL;
>> + } else {
>> + if (mem->userspace_addr)
>> + return -EINVAL;
>> + }
>> +
>> if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_MEM_SLOTS_NUM)
>> return -EINVAL;
>> if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
>> @@ -1328,7 +1347,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
>> } else { /* Modify an existing slot. */
>> if ((new.userspace_addr != old.userspace_addr) ||
>> (new.npages != old.npages) ||
>> - ((new.flags ^ old.flags) & KVM_MEM_READONLY))
>> + ((new.flags ^ old.flags) & KVM_MEM_READONLY) ||
>> + ((new.flags ^ old.flags) & KVM_MEM_PCI_HOLE))
>> return -EINVAL;
>>
>> if (new.base_gfn != old.base_gfn)
>> @@ -1715,13 +1735,13 @@ unsigned long kvm_host_page_size(struct kvm_vcpu *vcpu, gfn_t gfn)
>>
>> static bool memslot_is_readonly(struct kvm_memory_slot *slot)
>> {
>> - return slot->flags & KVM_MEM_READONLY;
>> + return slot->flags & (KVM_MEM_READONLY | KVM_MEM_PCI_HOLE);
>> }
>>
>> static unsigned long __gfn_to_hva_many(struct kvm_memory_slot *slot, gfn_t gfn,
>> gfn_t *nr_pages, bool write)
>> {
>> - if (!slot || slot->flags & KVM_MEMSLOT_INVALID)
>> + if (!slot || (slot->flags & (KVM_MEMSLOT_INVALID | KVM_MEM_PCI_HOLE)))
>> return KVM_HVA_ERR_BAD;
>>
>> if (memslot_is_readonly(slot) && write)
>> @@ -2318,6 +2338,11 @@ static int __kvm_read_guest_page(struct kvm_memory_slot *slot, gfn_t gfn,
>> int r;
>> unsigned long addr;
>>
>> + if (unlikely(slot && (slot->flags & KVM_MEM_PCI_HOLE))) {
>> + memset(data, 0xff, len);
>> + return 0;
>> + }
>> +
>> addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
>> if (kvm_is_error_hva(addr))
>> return -EFAULT;
>> --
>> 2.25.4
>>
>
> I didn't really review this patch, as it's touching lots of x86 mm
> functions that I didn't want to delve into, but I took a quick look
> since I was curious about the feature.

x86 part is really negligible, I think it would be very easy to expand
the scope to other arches if needed.

Thanks!

--
Vitaly

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