linux-kernel - Re: [PATCH 19/22] KVM: x86/mmu: Add infrastructure to allow walking rmaps outside of mmu

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Message-ID: <CAJhGHyDjsmQOQQoU52vA95sddWtzg1wh139jpPYBT1miUAgj6Q@mail.gmail.com>
Date: Mon, 12 Aug 2024 16:39:21 +0800
From: Lai Jiangshan <jiangshanlai@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>, Peter Xu <peterx@...hat.com>, 
	James Houghton <jthoughton@...gle.com>
Subject: Re: [PATCH 19/22] KVM: x86/mmu: Add infrastructure to allow walking
 rmaps outside of mmu_lock

On Sat, Aug 10, 2024 at 3:49 AM Sean Christopherson <seanjc@...gle.com> wrote:

> +
> +static unsigned long kvm_rmap_lock(struct kvm_rmap_head *rmap_head)
> +{
> +       unsigned long old_val, new_val;
> +
> +       old_val = READ_ONCE(rmap_head->val);
> +       if (!old_val)
> +               return 0;
> +
> +       do {
> +               /*
> +                * If the rmap is locked, wait for it to be unlocked before
> +                * trying acquire the lock, e.g. to bounce the cache line.
> +                */
> +               while (old_val & KVM_RMAP_LOCKED) {
> +                       old_val = READ_ONCE(rmap_head->val);
> +                       cpu_relax();

The sequence of these two lines of code can be improved.

> +               }
> +
> +               /*
> +                * Recheck for an empty rmap, it may have been purged by the
> +                * task that held the lock.
> +                */
> +               if (!old_val)
> +                       return 0;
> +
> +               new_val = old_val | KVM_RMAP_LOCKED;
> +       } while (!try_cmpxchg(&rmap_head->val, &old_val, new_val));
> +
> +       /* Return the old value, i.e. _without_ the LOCKED bit set. */
> +       return old_val;
> +}