linux-kernel - Re: [PATCH] KVM: x86: Use gfn_to_pfn_cache for steal

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Message-ID: <Zr_tRjKgPtk-uHjq@google.com>
Date: Fri, 16 Aug 2024 17:22:30 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Carsten Stollmaier <stollmc@...zon.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, nh-open-source@...zon.com, Peter Xu <peterx@...hat.com>, 
	Sebastian Biemueller <sbiemue@...zon.de>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Matthew Wilcox <willy@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] KVM: x86: Use gfn_to_pfn_cache for steal_time

On Fri, Aug 02, 2024, David Woodhouse wrote:
> On Fri, 2024-08-02 at 11:44 +0000, Carsten Stollmaier wrote:
> > On vcpu_run, before entering the guest, the update of the steal time
> > information causes a page-fault if the page is not present. In our
> > scenario, this gets handled by do_user_addr_fault and successively
> > handle_userfault since we have the region registered to that.
> > 
> > handle_userfault uses TASK_INTERRUPTIBLE, so it is interruptible by
> > signals. do_user_addr_fault then busy-retries it if the pending signal
> > is non-fatal. This leads to contention of the mmap_lock.
> 
> The busy-loop causes so much contention on mmap_lock that post-copy
> live migration fails to make progress, and is leading to failures. Yes?
> 
> > This patch replaces the use of gfn_to_hva_cache with gfn_to_pfn_cache,
> > as gfn_to_pfn_cache ensures page presence for the memory access,
> > preventing the contention of the mmap_lock.
> > 
> > Signed-off-by: Carsten Stollmaier <stollmc@...zon.com>
> 
> Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>
> 
> I think this makes sense on its own, as it addresses the specific case
> where KVM is *likely* to be touching a userfaulted (guest) page. And it
> allows us to ditch yet another explicit asm exception handler.

At the cost of using a gpc, which has its own complexities.

But I don't understand why steal_time is special.  If the issue is essentially
with handle_userfault(), can't this happen on any KVM uaccess?