Re: [PATCH v2 08/10] KVM: selftests: create alias mappings when using shared memory

From: Axel Rasmussen
Date: Wed May 26 2021 - 13:23:40 EST

I applied this change on top of kvm/master and tested it, and indeed
it compiles and works correctly.

Paolo, feel free to take this with a:

Reviewed-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>

Or alternatively if you prefer I'm happy to send it as a real
git-send-email patch.

On Tue, May 25, 2021 at 4:50 PM David Matlack <dmatlack@xxxxxxxxxx> wrote:
>
> 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(&region->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
> >