linux-kernel - Re: [PATCH v12 02/84] KVM: arm64: Disallow copying MTE to guest memory while KVM is dirty logging

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Message-ID: <yq5a5xsftna9.fsf@kernel.org>
Date: Mon, 05 Aug 2024 13:27:18 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
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

Sean Christopherson <seanjc@...gle.com> writes:

> 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.
>


What I meant was, should we consider mte_copy_tags_from_user() as one
that update the page contents (even though it is updating tags) and
use kvm_follow_pfn() with kfp->pin = 1 instead?

Is my understanding correct in that, if we want to look up a pfn/page
from gfn with the intent of updating the page contents, we should use
kfp->pin == 1? 

-aneesh