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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

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.

- even if the MMIO region is not used by emulated-device, it also need to be
  registered.

It will breaks old version userspace program.

> exiting for each 8 bytes? Is it worth the complexity?

Simpler way is always better but i failed, so i appreciate your guys comments.