Re: [PATCH] KVM: x86: Use gfn_to_pfn_cache for steal_time

[Sean Christopherson <seanjc@google.com>]

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@amazon.com>
> 
> Reviewed-by: David Woodhouse <dwmw@amazon.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?