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

[David Woodhouse <dwmw2@infradead.org>]

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On Tue, 2024-08-20 at 11:11 +0100, David Woodhouse wrote:
> On Fri, 2024-08-16 at 17:22 -0700, Sean Christopherson wrote:
> > 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?
> 
> Theoretically, yes. The steal time is only special in that it happens
> so *often*, every time the vCPU is scheduled in.
> 
> We should *also* address the general case, perhaps making by
> interruptible user access functions as discussed. But this solves the
> immediate issue which is being observed, *and* lets us ditch the last
> explicit asm exception handling in kvm/x86.c which is why I think it's
> worth doing anyway, even if there's an upcoming fix for the general
> case.

Gentle reader, there was *not* an upcoming fix for the general case,
and this is still an issue in real-world use cases... or would be, if
we hadn't been using Carsten's patch ever since he came up with it.

I still quite like the idea of interruptible user access, but nobody
seemed that excited by it, and as noted I think we should move steal
time to a GPC *anyway*.


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