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

From: Jan Kiszka
Date: Mon Oct 22 2012 - 07:53:11 EST

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.


Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux
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