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@xxxxxxxxxx> > > > > Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx> > > > > 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.
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