On 16/08/2017 14:58, Michael S. Tsirkin wrote:
> On Wed, Aug 16, 2017 at 01:22:49PM +0200, Paolo Bonzini wrote:
>> Microsoft pointed out privately to me that KVM's handling of
>> KVM_FAST_MMIO_BUS is invalid. Using skip_emulation_instruction is invalid
>> in EPT misconfiguration vmexit handlers, because neither EPT violations
>> nor misconfigurations are listed in the manual among the VM exits that
>> set the VM-exit instruction length field.
>>
>> While physical processors seem to set the field, this is not architectural
>> and is just a side effect of the implementation. I couldn't convince
>> myself of any condition on the exit qualification where VM-exit
>> instruction length "has" to be defined; there are no trap-like VM-exits
>> that can be repurposed; and fault-like VM-exits such as descriptor-table
>> exits provide no decoding information. So I don't really see any elegant
>> way to fix it except by disabling KVM_FAST_MMIO_BUS, which means virtio
>> 1 will go slower.
>
> How about I will try asking Intel about it? If they can commit to length
> being there in the future, we are all set.
Nope, "I couldn't convince myself of any condition on the exit
qualification where VM-exit instruction length "has" to be defined". So
assuming Intel can do it, it would only apply to future processors (2
years+ for server SKUs).
Plus of course it wouldn't be guaranteed to work on nested.
>> Adding a hypercall or MSR write that does a fast MMIO write to a physical
>> address would do it, but it adds hypervisor knowledge in virtio, including
>> CPUID handling.
>
> Another issue is that it will break DPDK on virtio.
Not break, just make it slower.
Paolo
> Hmm that's quite unfortunate as we have just completed rolling out MMIO
> signalling across the board. We did measure a significant slowdown
> before enabling fast mmio.
>
> Guest TX:(TCP)
> size/session/+throughput%/+cpu%/-+per cpu%/
> 64/1/[+18.9183%]/-0.2823%/[+19.2550%]/
> 64/2/[+13.5714%]/[+2.2675%]/[+11.0533%]/
> 64/4/[+13.1070%]/[+2.1817%]/[+10.6920%]/
> 64/8/[+13.0426%]/[+2.0887%]/[+10.7299%]/
> 256/1/[+36.2761%]/+6.3434%/[+28.1471%]/
> ...
> 1024/1/[+44.8873%]/+2.0811%/[+41.9335%]/
> ...
> 1024/4/+0.0228%/[-2.2044%]/[+2.2774%]/
> ...
> 16384/2/+0.0127%/[-5.0346%]/[+5.3148%]/
> ...
> 65535/1/[+0.0062%]/[-4.1183%]/[+4.3017%]/
> 65535/2/+0.0004%/[-4.2311%]/[+4.4185%]/
> 65535/4/+0.0107%/[-4.6106%]/[+4.8446%]/
> 65535/8/-0.0090%/[-5.5178%]/[+5.8306%]/
>
>
> See commit bc85ccfdf5cc045588f665c84b5707d7364c8a6c for more numbers.
>
>
>> ---
>> arch/x86/kvm/vmx.c | 5 -----
>> 1 file changed, 5 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 375dca24cf42..b3eaeb20670d 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -6320,11 +6320,6 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
>> gpa_t gpa;
>>
>> gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
>> - if (!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
>> - trace_kvm_fast_mmio(gpa);
>> - return kvm_skip_emulated_instruction(vcpu);
>> - }
>> -
>> ret = handle_mmio_page_fault(vcpu, gpa, true);
>> vcpu->arch.gpa_available = true;
>> if (likely(ret == RET_MMIO_PF_EMULATE))
>> --
>> 2.13.5
