linux-kernel - Re: [EXTERNAL] [PATCH v2] KVM: Don't actually set a request when evicting vCPUs for GFN cache invd

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Date:   Fri, 25 Feb 2022 16:59:59 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "borntraeger@...ux.ibm.com" <borntraeger@...ux.ibm.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "frankja@...ux.ibm.com" <frankja@...ux.ibm.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "imbrenda@...ux.ibm.com" <imbrenda@...ux.ibm.com>,
        "david@...hat.com" <david@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [EXTERNAL] [PATCH v2] KVM: Don't actually set a request when
 evicting vCPUs for GFN cache invd

On Fri, 2022-02-25 at 16:13 +0000, Sean Christopherson wrote:
> On Fri, Feb 25, 2022, Woodhouse, David wrote:
> > Since we need an active vCPU context to do dirty logging (thanks, dirty
> > ring)... and since any time vcpu_run exits to userspace for any reason
> > might be the last time we ever get an active vCPU context... I think
> > that kind of fundamentally means that we must flush dirty state to the
> > log on *every* return to userspace, doesn't it?
> 
> I would rather add a variant of mark_page_dirty_in_slot() that takes a vCPU, which
> we whould have in all cases.  I see no reason to require use of kvm_get_running_vcpu().

We already have kvm_vcpu_mark_page_dirty(), but it can't use just 'some
vcpu' because the dirty ring is lockless. So if you're ever going to
use anything other than kvm_get_running_vcpu() we need to add locks.

And while we *could* do that, I don't think it would negate the
fundamental observation that *any* time we return from vcpu_run to
userspace, that could be the last time. Userspace might read the dirty
log for the *last* time, and any internally-cached "oh, at some point
we need to mark <this> page dirty" is lost because by the time the vCPU
is finally destroyed, it's too late.

I think I'm going to rip out the 'dirty' flag from the gfn_to_pfn_cache
completely and add a function (to be called with an active vCPU
context) which marks the page dirty *now*.

KVM_GUEST_USES_PFN users like nested VMX will be expected to do this
before returning from vcpu_run anytime it's in L2 guest mode. 

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