On 2012-10-22 13:43, Gleb Natapov wrote: > 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)? Yes. We only have a single ring per VM, so we cannot flush multi-second VGA access separately from other devices. In theory solvable by introducing per-region rings that can be driven separately. 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
