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

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Message-ID: <ZrooozABEWSnwzxh@google.com>
Date: Mon, 12 Aug 2024 08:22:11 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Lai Jiangshan <jiangshanlai@...il.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 Mon, Aug 12, 2024, Lai Jiangshan wrote:
> 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.

Oh yeah, duh, re-read after PAUSE, not before.

Definitely holler if you have any alternative ideas for walking rmaps without
taking mmu_lock, I guarantee you've spent more time than me thinking about the
shadow MMU :-)