Re: [RFC V1 PATCH 0/5] selftests: KVM: selftests for fd-based approach of supporting private memory

[Chao Peng]

On Mon, Apr 11, 2022 at 05:31:09PM +0530, Nikunj A. Dadhania wrote:
> On 4/9/2022 2:35 AM, Vishal Annapurve wrote:
> > This series implements selftests targeting the feature floated by Chao
> > via:
> > https://lore.kernel.org/linux-mm/20220310140911.50924-1-chao.p.peng@xxxxxxxxxxxxxxx/
> > 
> 
> Thanks for working on this.
> 
> > Below changes aim to test the fd based approach for guest private memory
> > in context of normal (non-confidential) VMs executing on non-confidential
> > platforms.
> > 
> > Confidential platforms along with the confidentiality aware software
> > stack support a notion of private/shared accesses from the confidential
> > VMs.
> > Generally, a bit in the GPA conveys the shared/private-ness of the
> > access. Non-confidential platforms don't have a notion of private or
> > shared accesses from the guest VMs. To support this notion,
> > KVM_HC_MAP_GPA_RANGE
> > is modified to allow marking an access from a VM within a GPA range as
> > always shared or private. Any suggestions regarding implementing this ioctl
> > alternatively/cleanly are appreciated.
> > 
> > priv_memfd_test.c file adds a suite of two basic selftests to access private
> > memory from the guest via private/shared access and checking if the contents
> > can be leaked to/accessed by vmm via shared memory view.
> > 
> > Test results:
> > 1) PMPAT - PrivateMemoryPrivateAccess test passes
> > 2) PMSAT - PrivateMemorySharedAccess test fails currently and needs more
> > analysis to understand the reason of failure.
> 
> That could be because of the return code (*r = -1) from the KVM_EXIT_MEMORY_ERROR. 
> This gets interpreted as -EPERM in the VMM when the vcpu_run exits.
> 
> 	+	vcpu->run->exit_reason = KVM_EXIT_MEMORY_ERROR;
> 	+	vcpu->run->memory.flags = flags;
> 	+	vcpu->run->memory.padding = 0;
> 	+	vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
> 	+	vcpu->run->memory.size = PAGE_SIZE;
> 	+	fault->pfn = -1;
> 	+	*r = -1;
> 	+	return true;

That's true. The current private mem patch treats KVM_EXIT_MEMORY_ERROR as error
for KVM_RUN. That behavior needs to be discussed, but right now (v5) it hits the
ASSERT in tools/testing/selftests/kvm/lib/kvm_util.c before you have chance to
handle KVM_EXIT_MEMORY_ERROR in this patch series.

void vcpu_run(struct kvm_vm *vm, uint32_t vcpuid)
{
        int ret = _vcpu_run(vm, vcpuid);
        TEST_ASSERT(ret == 0, "KVM_RUN IOCTL failed, "
                "rc: %i errno: %i", ret, errno);
}

Thanks,
Chao

> 
> 
> Regards
> Nikunj
> 
> [1] https://lore.kernel.org/all/20220310140911.50924-10-chao.p.peng@xxxxxxxxxxxxxxx/#t