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

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 02, 2024 at 01:03:16PM +0100, 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.
> 
> We should note, though, that in terms of the original problem described
> above, it's a bit of a workaround. It just means that by using
> kvm_gpc_refresh() to obtain the user page, we end up in
> handle_userfault() without the FAULT_FLAG_INTERRUPTIBLE flag.
> 
> (Note to self: should kvm_gpc_refresh() take fault flags, to allow
> interruptible and killable modes to be selected by its caller?)
> 
> 
> An alternative workaround (which perhaps we should *also* consider)
> looked like this (plus some suitable code comment, of course):
> 
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1304,6 +1304,8 @@ void do_user_addr_fault(struct pt_regs *regs,
>          */
>         if (user_mode(regs))
>                 flags |= FAULT_FLAG_USER;
> +       else
> +               flags &= ~FAULT_FLAG_INTERRUPTIBLE;
>  
>  #ifdef CONFIG_X86_64
>         /*
> 
> 
> That would *also* handle arbitrary copy_to_user/copy_from_user() to
> userfault pages, which could theoretically hit the same busy loop.
> 
> I'm actually tempted to make user access *interruptible* though, and
> either add copy_{from,to}_user_interruptible() or change the semantics
> of the existing ones (which I believe are already killable).
> 
> That would require each architecture implementing interruptible
> exceptions, by doing an extable lookup before the retry. Not overly
> complex, but needs to be done for all architectures (although not at
> once; we could live with not-yet-done architectures just remaining
> killable).
> 
> Thoughts?

Instead of "interruptible exception" or the original patch (which might
still be worthwhile, though?  I didn't follow much on kvm and the new gpc
cache, but looks still nicer than get/put user from initial glance), above
looks like the easier and complete solution to me.  For "completeness", I
mean I am not sure how many other copy_to/from_user() code in kvm can hit
this, so looks like still possible to hit outside steal time page?

I thought only the slow fault path was involved in INTERRUPTIBLE thing and
that was the plan, but I guess I overlooked how the default value could
affect copy to/from user invoked from KVM as well..

With above patch to drop FAULT_FLAG_INTERRUPTIBLE for !user, KVM can still
opt-in INTERRUPTIBLE anywhere by leveraging hva_to_pfn[_slow]() API, which
is "INTERRUPTIBLE"-ready with a boolean the caller can set. But the caller
will need to be able to process KVM_PFN_ERR_SIGPENDING.

Thanks,

-- 
Peter Xu