Re: [PATCH] KVM: x86: fix vcpu->mmio_fragments overflow

[Gleb Natapov <gleb@redhat.com>]

On Mon, Oct 22, 2012 at 01:35:56PM +0200, Jan Kiszka wrote:
> On 2012-10-22 13:23, Gleb Natapov wrote:
> > On Mon, Oct 22, 2012 at 07:09:38PM +0800, Xiao Guangrong wrote:
> >> On 10/22/2012 05:16 PM, Gleb Natapov wrote:
> >>> On Fri, Oct 19, 2012 at 03:37:32PM +0800, Xiao Guangrong wrote:
> >>>> After commit b3356bf0dbb349 (KVM: emulator: optimize "rep ins" handling),
> >>>> the pieces of io data can be collected and write them to the guest memory
> >>>> or MMIO together.
> >>>>
> >>>> Unfortunately, kvm splits the mmio access into 8 bytes and store them to
> >>>> vcpu->mmio_fragments. If the guest uses "rep ins" to move large data, it
> >>>> will cause vcpu->mmio_fragments overflow
> >>>>
> >>>> The bug can be exposed by isapc (-M isapc):
> >>>>
> >>>> [23154.818733] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
> >>>> [ ......]
> >>>> [23154.858083] Call Trace:
> >>>> [23154.859874]  [<ffffffffa04f0e17>] kvm_get_cr8+0x1d/0x28 [kvm]
> >>>> [23154.861677]  [<ffffffffa04fa6d4>] kvm_arch_vcpu_ioctl_run+0xcda/0xe45 [kvm]
> >>>> [23154.863604]  [<ffffffffa04f5a1a>] ? kvm_arch_vcpu_load+0x17b/0x180 [kvm]
> >>>>
> >>>>
> >>>> Actually, we can use one mmio_fragment to store a large mmio access for the
> >>>> mmio access is always continuous then split it when we pass the mmio-exit-info
> >>>> to userspace. After that, we only need two entries to store mmio info for
> >>>> the cross-mmio pages access
> >>>>
> >>> I wonder can we put the data into coalesced mmio buffer instead of
> >>
> >> If we put all mmio data into coalesced buffer, we should:
> >> - ensure the userspace program uses KVM_REGISTER_COALESCED_MMIO to register
> >>   all mmio regions.
> >>
> > It appears to not be so.
> > Userspace calls kvm_flush_coalesced_mmio_buffer() after returning from
> > KVM_RUN which looks like this:
> 
> Nope, no longer, only on accesses to devices that actually use such
> regions (and there are only two ATM). The current design of a global
> coalesced mmio ring is horrible /wrt latency.
> 
Indeed. git pull, recheck and call for kvm_flush_coalesced_mmio_buffer()
is gone. So this will break new userspace, not old. By global you mean
shared between devices (or memory regions)?

--
			Gleb.