Re: [PATCH v2 08/10] KVM: selftests: create alias mappings when using shared memory
From: David Matlack
Date: Tue May 25 2021 - 19:50:09 EST
On Wed, May 19, 2021 at 1:04 PM Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote:
>
> When a memory region is added with a src_type specifying that it should
> use some kind of shared memory, also create an alias mapping to the same
> underlying physical pages.
>
> And, add an API so tests can get access to these alias addresses.
> Basically, for a guest physical address, let us look up the analogous
> host *alias* address.
>
> In a future commit, we'll modify the demand paging test to take
> advantage of this to exercise UFFD minor faults. The idea is, we
> pre-fault the underlying pages *via the alias*. When the *guest*
> faults, it gets a "minor" fault (PTEs don't exist yet, but a page is
> already in the page cache). Then, the userfaultfd theads can handle the
> fault: they could potentially modify the underlying memory *via the
> alias* if they wanted to, and then they install the PTEs and let the
> guest carry on via a UFFDIO_CONTINUE ioctl.
>
> Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
> Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> ---
> .../testing/selftests/kvm/include/kvm_util.h | 1 +
> tools/testing/selftests/kvm/lib/kvm_util.c | 51 +++++++++++++++++++
> .../selftests/kvm/lib/kvm_util_internal.h | 2 +
> 3 files changed, 54 insertions(+)
>
> diff --git a/tools/testing/selftests/kvm/include/kvm_util.h b/tools/testing/selftests/kvm/include/kvm_util.h
> index a8f022794ce3..0624f25a6803 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util.h
> @@ -146,6 +146,7 @@ void virt_map(struct kvm_vm *vm, uint64_t vaddr, uint64_t paddr,
> void *addr_gpa2hva(struct kvm_vm *vm, vm_paddr_t gpa);
> void *addr_gva2hva(struct kvm_vm *vm, vm_vaddr_t gva);
> vm_paddr_t addr_hva2gpa(struct kvm_vm *vm, void *hva);
> +void *addr_gpa2alias(struct kvm_vm *vm, vm_paddr_t gpa);
>
> /*
> * Address Guest Virtual to Guest Physical
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index e4a8d0c43c5e..0b88d1bbc1e0 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -811,6 +811,19 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
>
> /* Add to linked-list of memory regions. */
> list_add(®ion->list, &vm->userspace_mem_regions);
> +
> + /* If shared memory, create an alias. */
> + if (region->fd >= 0) {
> + region->mmap_alias = mmap(NULL, region->mmap_size,
> + PROT_READ | PROT_WRITE,
> + vm_mem_backing_src_alias(src_type)->flag,
> + region->fd, 0);
> + TEST_ASSERT(region->mmap_alias != MAP_FAILED,
> + "mmap of alias failed, errno: %i", errno);
> +
> + /* Align host alias address */
> + region->host_alias = align(region->mmap_alias, alignment);
> + }
> }
>
> /*
> @@ -1239,6 +1252,44 @@ vm_paddr_t addr_hva2gpa(struct kvm_vm *vm, void *hva)
> return -1;
> }
>
> +/*
> + * Address VM physical to Host Virtual *alias*.
> + *
> + * Input Args:
> + * vm - Virtual Machine
> + * gpa - VM physical address
> + *
> + * Output Args: None
> + *
> + * Return:
> + * Equivalent address within the host virtual *alias* area, or NULL
> + * (without failing the test) if the guest memory is not shared (so
> + * no alias exists).
> + *
> + * When vm_create() and related functions are called with a shared memory
> + * src_type, we also create a writable, shared alias mapping of the
> + * underlying guest memory. This allows the host to manipulate guest memory
> + * without mapping that memory in the guest's address space. And, for
> + * userfaultfd-based demand paging, we can do so without triggering userfaults.
> + */
> +void *addr_gpa2alias(struct kvm_vm *vm, vm_paddr_t gpa)
> +{
> + struct userspace_mem_region *region;
> +
> + list_for_each_entry(region, &vm->userspace_mem_regions, list) {
This patch fails to compile on top of with db0670ce3361 ("KVM:
selftests: Keep track of memslots more efficiently").
This can be reproduced by checking out kvm/master and running `make -C
tools/testing/selftests/kvm`.
The following diff fixes the compilation error but I did not have time
to test it yet:
diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c
b/tools/testing/selftests/kvm/lib/kvm_util.c
index c98db1846e1b..28e528c19d28 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -1374,19 +1374,17 @@ vm_paddr_t addr_hva2gpa(struct kvm_vm *vm, void *hva)
void *addr_gpa2alias(struct kvm_vm *vm, vm_paddr_t gpa)
{
struct userspace_mem_region *region;
+ uintptr_t offset;
- list_for_each_entry(region, &vm->userspace_mem_regions, list) {
- if (!region->host_alias)
- continue;
+ region = userspace_mem_region_find(vm, gpa, gpa);
+ if (!region)
+ return NULL;
- if ((gpa >= region->region.guest_phys_addr)
- && (gpa <= (region->region.guest_phys_addr
- + region->region.memory_size - 1)))
- return (void *) ((uintptr_t) region->host_alias
- + (gpa - region->region.guest_phys_addr));
- }
+ if (!region->host_alias)
+ return NULL;
- return NULL;
+ offset = gpa - region->region.guest_phys_addr;
+ return (void *) ((uintptr_t) region->host_alias + offset);
}
/*
> + if (!region->host_alias)
> + continue;
> +
> + if ((gpa >= region->region.guest_phys_addr)
> + && (gpa <= (region->region.guest_phys_addr
> + + region->region.memory_size - 1)))
> + return (void *) ((uintptr_t) region->host_alias
> + + (gpa - region->region.guest_phys_addr));
> + }
> +
> + return NULL;
> +}
> +
> /*
> * VM Create IRQ Chip
> *
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util_internal.h b/tools/testing/selftests/kvm/lib/kvm_util_internal.h
> index 91ce1b5d480b..a25af33d4a9c 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util_internal.h
> +++ b/tools/testing/selftests/kvm/lib/kvm_util_internal.h
> @@ -16,7 +16,9 @@ struct userspace_mem_region {
> int fd;
> off_t offset;
> void *host_mem;
> + void *host_alias;
> void *mmap_start;
> + void *mmap_alias;
> size_t mmap_size;
> struct list_head list;
> };
> --
> 2.31.1.751.gd2f1c929bd-goog
>