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Message-ID: <ZqvNekQAjs-SN-se@google.com>
Date: Thu, 1 Aug 2024 11:01:30 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>, Tianrui Zhao <zhaotianrui@...ngson.cn>,
Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>,
Michael Ellerman <mpe@...erman.id.au>, Anup Patel <anup@...infault.org>,
Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
David Matlack <dmatlack@...gle.com>, David Stevens <stevensd@...omium.org>
Subject: Re: [PATCH v12 02/84] KVM: arm64: Disallow copying MTE to guest
memory while KVM is dirty logging
On Thu, Aug 01, 2024, Aneesh Kumar K.V wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
>
> > Disallow copying MTE tags to guest memory while KVM is dirty logging, as
> > writing guest memory without marking the gfn as dirty in the memslot could
> > result in userspace failing to migrate the updated page. Ideally (maybe?),
> > KVM would simply mark the gfn as dirty, but there is no vCPU to work with,
> > and presumably the only use case for copy MTE tags _to_ the guest is when
> > restoring state on the target.
> >
> > Fixes: f0376edb1ddc ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> > arch/arm64/kvm/guest.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index e1f0ff08836a..962f985977c2 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -1045,6 +1045,11 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> >
> > mutex_lock(&kvm->slots_lock);
> >
> > + if (write && atomic_read(&kvm->nr_memslots_dirty_logging)) {
> > + ret = -EBUSY;
> > + goto out;
> > + }
> > +
> >
>
> is this equivalent to kvm_follow_pfn() with kfp->pin = 1 ?
No, gfn_to_pfn_prot() == FOLL_GET, kfp->pin == FOLL_PIN. But that's not really
relevant.
> Should all those pin request fail if kvm->nr_memslots_dirty_logging != 0?
No, the conflict with dirty logging is specifically that this code doesn't invoke
mark_page_dirty(). And it can't easily do that, because there's no loaded ("running")
vCPU, i.e. doing so would trip this WARN:
#ifdef CONFIG_HAVE_KVM_DIRTY_RING
if (WARN_ON_ONCE(vcpu && vcpu->kvm != kvm))
return;
WARN_ON_ONCE(!vcpu && !kvm_arch_allow_write_without_running_vcpu(kvm)); <====
#endif
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