Re: [PATCH v2 11/27] KVM: x86/mmu: Zap only the relevant pages when removing a memslot

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	kvm@vger.kernel.org, "Xiao Guangrong" <guangrong.xiao@gmail.com>
Subject: Re: [PATCH v2 11/27] KVM: x86/mmu: Zap only the relevant pages when removing a memslot
Date: Thu, 22 Aug 2019 19:25:02 -0700	[thread overview]
Message-ID: <20190823022502.GA4525@linux.intel.com> (raw)
In-Reply-To: <20190821203041.GJ29345@linux.intel.com>

On Wed, Aug 21, 2019 at 01:30:41PM -0700, Sean Christopherson wrote:
> On Wed, Aug 21, 2019 at 01:35:04PM -0600, Alex Williamson wrote:
> > On Wed, 21 Aug 2019 13:08:59 -0600
> > Alex Williamson <alex.williamson@redhat.com> wrote:
> > > Does this suggests something is still fundamentally wrong with the
> > > premise of this change or have I done something stupid?
> > 
> > Seems the latter, particularly your comment that we're looking for
> > pages pointing to the gfn range to be removed, not just those in the
> > range.  Slot gfn ranges like ffe00-ffe1f are getting reduced to 0-0 or
> > c0000-c0000, zapping zero or c0000, and I think one of the ones you
> > were looking for c1080-c1083 is reduce to c1000-c1000 and therefore
> > zaps sp->gfn c1000.  I'll keep looking.  Thanks,
> 
> Ya.  As far as where to look, at this point I don't think it's an issue of
> incorrect zapping.  Not because  I'm 100% confident the zapping logic is
> correct, but because many of the tests, e.g. removing 'sp->gfn != gfn' and
> not being able to exclude APIC/IOAPIC ranges, suggest that the badness is
> 'fixed' by zapping seemingly unrelated sps.
> 
> In other words, it may be fundamentally wrong to zap only the memslot
> being removed, but I really want to know why.  History isn't helpful as
> KVM has always zapped all pages when removing a memslot (on x86), and the
> introduction of the per-memslot flush hook in commit
> 
>   2df72e9bc4c5 ("KVM: split kvm_arch_flush_shadow")
> 
> was all about refactoring generic code, and doesn't have any information
> on whether per-memslot flushing was actually tried for x86.

One semi-random idea would be to zap mmio pages, i.e. don't skip pages
for which sp->mmio_cached is true, regardless of their gfn or level.  I
don't expect it to make a difference, but it would shrink the haystack on
the off change it does "fix" the issues.