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. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
