linux-kernel - Re: [PATCH gmem] KVM: selftests: Fix gmem conversion tests for multiple vCPUs

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Date:   Mon, 23 Oct 2023 11:06:49 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Michael Roth <michael.roth@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, vannapurve@...gle.com
Subject: Re: [PATCH gmem] KVM: selftests: Fix gmem conversion tests for
 multiple vCPUs

On Tue, Oct 17, 2023, Sean Christopherson wrote:
> On Mon, Oct 16, 2023, Michael Roth wrote:
> > +			vm_mem_add(vm, src_type, BASE_DATA_GPA + size * i,
> > +				   BASE_DATA_SLOT + i, size / vm->page_size,
> > +				   KVM_MEM_PRIVATE, memfd, size * i);
> 
> But I think that's a moot point, because isn't it easier to do this?
> 
> diff --git a/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c b/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
> index c04e7d61a585..c99073098f98 100644
> --- a/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
> @@ -367,6 +367,7 @@ static void test_mem_conversions(enum vm_mem_backing_src_type src_type, uint32_t
>          */
>         const size_t size = align_up(PER_CPU_DATA_SIZE, get_backing_src_pagesz(src_type));
>         const size_t memfd_size = size * nr_vcpus;
> +       const size_t slot_size = memfd_size / nr_memslots;
>         struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
>         pthread_t threads[KVM_MAX_VCPUS];
>         uint64_t gmem_flags;
> @@ -390,7 +391,7 @@ static void test_mem_conversions(enum vm_mem_backing_src_type src_type, uint32_t
>  
>         for (i = 0; i < nr_memslots; i++)
>                 vm_mem_add(vm, src_type, BASE_DATA_GPA + size * i,
> -                          BASE_DATA_SLOT + i, size / vm->page_size,
> +                          BASE_DATA_SLOT + i, slot_size / vm->page_size,
>                            KVM_MEM_PRIVATE, memfd, size * i);

This isn't quite right, the stride and offset needs to be per-memslot too.  Argh,
I created quite the mess by trying to take a shortcut for testing multiple memslots,
i.e. by only allowing '1' or "nr_vcpus" memslots.  Much of the code assumes that
ranges can't be covered by multiple memslots, e.g. the UCALL_SYNC handler assumes
the entire range is contiguous in the host virtual address space.

And I think there's meaningful coverage we're not getting, e.g. as is I don't think
we're testing KVM_SET_MEMORY_ATTRIBUTES across multiple memslots (thankfully we
seem to have gotten the KVM side of things correct).

I'll post a small series to clean up the mess and let the user specify the number
of memslots (with some restrictions to keep the code relatively simple).