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

From: Aneesh Kumar K.V <[email-masked]>
Date: Mon, 05 Aug 2024 13:27:18 +0530

Sean Christopherson <[email-masked]> writes:

> On Thu, Aug 01, 2024, Aneesh Kumar K.V wrote:
>> Sean Christopherson <[email-masked]> 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 <[email-masked]>
>> > ---
>> >  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