linux-kernel - Re: [PATCH] KVM: selftests: fix supported

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Message-ID: <ZXnoCadq2J3cPz2r@google.com>
Date:   Wed, 13 Dec 2023 09:21:13 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Shaoqin Huang <shahuang@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: selftests: fix supported_flags for aarch64

On Tue, Dec 12, 2023, Paolo Bonzini wrote:
> On 12/9/23 03:29, Sean Christopherson wrote:
> > On Fri, Dec 08, 2023, Paolo Bonzini wrote:
> > > KVM/Arm supports readonly memslots; fix the calculation of
> > > supported_flags in set_memory_region_test.c, otherwise the
> > > test fails.
> > 
> > You got beat by a few hours, and by a better solution ;-)
> > 
> > https://lore.kernel.org/all/20231208033505.2930064-1-shahuang@redhat.com
> 
> Better but also wrong---and my patch has the debatable merit of more
> clearly exposing the wrongness.  Testing individual architectures is bad,
> but testing __KVM_HAVE_READONLY_MEM makes the test fail when running a new
> test on an old kernel.

But we already crossed that bridge and burned it for good measure by switching
to KVM_SET_USER_MEMORY_REGION2, i.e. as of commit

  8d99e347c097 ("KVM: selftests: Convert lib's mem regions to KVM_SET_USER_MEMORY_REGION2")

selftests built against a new kernel can't run on an old kernel.  Building KVM
selftests requires kernel headers, so while not having a hard requirement that
the uapi headers are fresh would be nice, I don't think it buys all that much.

If we wanted to assert that x86, arm64, etc. enumerate __KVM_HAVE_READONLY_MEM,
i.e. ensure that read-only memory is supported as expected, then that can be done
as a completely unrelated test.

IMO, one of the big selling points of selftests over KUT is that we can punt on
supporting old kernels since selftests are in-tree.  I don't think it's at all
unreasonable to require that selftests be built against the target kernel, and
by doing so we can signficantly reduce the maintenance burden.  The kernel needs
to be backwards compatibile, but I don't see why selftests need to be backwards
compatible.