linux-kernel - Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY

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Message-ID: <aPu7IosMI61NjZY5@google.com>
Date: Fri, 24 Oct 2025 10:45:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2

On Fri, Oct 24, 2025, Ackerley Tng wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> > @@ -486,6 +488,7 @@ struct kvm_vm *__vm_create(struct vm_shape shape, uint32_t nr_runnable_vcpus,
> >         }
> >         guest_rng = new_guest_random_state(guest_random_seed);
> >         sync_global_to_guest(vm, guest_rng);
> > +       sync_global_to_guest(vm, kvm_has_gmem_attributes);
> 
> I ported this [1] except for syncing this value to the guest, because I
> think the guest shouldn't need to know this information,

KVM selftests are about practically and testing, what information should or
shouldn't be available to a test from e.g. a safety perspective is completely
irrelevant.  In fact, one of the biggest advantages of selftests over KUT is
that the guest side can know _exactly_ what's going on in the host.

See the usage in 1850e3da4b03 ("KVM: selftests: Update private_mem_conversions_test
to mmap() guest_memfd") from:

  https://github.com/sean-jc/linux.git x86/gmem_inplace

> the host should decide what to do. I think, if the guests really need to know
> this, the test itself can do the syncing.

Why force tests to do extra work, and potentially introduce subtle bugs due to
state being stale?