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. -- 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
