linux-kernel - Re: [PATCH v4] kvm: better MWAIT emulation for guests

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Date:   Thu, 16 Mar 2017 17:30:06 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Gabriel L. Somlo" <gsomlo@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v4] kvm: better MWAIT emulation for guests

2017-03-16 4:13 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-03-15 21:28+0200, Michael S. Tsirkin:
>> Guests running Mac OS 5, 6, and 7 (Leopard through Lion) have a problem:
>> unless explicitly provided with kernel command line argument
>> "idlehalt=0" they'd implicitly assume MONITOR and MWAIT availability,
>> without checking CPUID.
>>
>> We currently emulate that as a NOP but on VMX we can do better: let
>> guest stop the CPU until timer, IPI or memory change.  CPU will be busy
>> but that isn't any worse than a NOP emulation.
>>
>> Note that mwait within guests is not the same as on real hardware
>> because halt causes an exit while mwait doesn't.  For this reason it
>> might not be a good idea to use the regular MWAIT flag in CPUID to
>> signal this capability.  Add a flag in the hypervisor leaf instead.
>>
>> Additionally, we add a capability for QEMU - e.g. if it knows there's an
>> isolated CPU dedicated for the VCPU it can set the standard MWAIT flag
>> to improve guest behaviour.
>>
>> Reported-by: "Gabriel L. Somlo" <gsomlo@...il.com>
>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>> ---
>>
>> Note: SVM bits are untested at this point. Seems pretty
>> obvious though.
>>
>> changes from v3:
>> - don't enable capability if cli+mwait blocks interrupts
>> - doc typo fixes (drop drop ppc doc)
>>
>> changes from v2:
>> - add a capability to allow host userspace to detect new kernels
>> - more documentation to clarify the semantics of the feature flag
>>   and why it's useful
>> - svm support as suggested by Radim
>>
>> changes from v1:
>> - typo fix resulting in rest of leaf flags being overwritten
>>   Reported by: Wanpeng Li <kernellwp@...il.com>
>> - updated commit log with data about guests helped by this feature
>> - better document differences between mwait and halt for guests
>>
>> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
>> @@ -212,4 +213,28 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)
>>           __rem;                                              \
>>        })
>>
>> +static bool kvm_mwait_in_guest(void)
>> +{
>> +     unsigned int eax, ebx, ecx;
>> +
>> +     if (!cpu_has(&boot_cpu_data, X86_FEATURE_MWAIT))
>> +             return -ENODEV;
>> +
>> +     if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
>> +             return -ENODEV;
>> +
>> +     /*
>> +      * Intel CPUs without CPUID5_ECX_INTERRUPT_BREAK are problematic as
>> +      * they would allow guest to stop the CPU completely by disabling
>> +      * interrupts then invoking MWAIT.
>> +      */
>> +     if (boot_cpu_data.cpuid_level < CPUID_MWAIT_LEAF)
>> +             return -ENODEV;
>> +
>> +     cpuid(CPUID_MWAIT_LEAF, &eax, &ebx, &ecx, &mwait_substates);
>> +
>> +     if (!(ecx & CPUID5_ECX_INTERRUPT_BREAK))
>> +             return -ENODEV;
>
> The guest is still able to set ecx=0 with MWAIT, which should be the

How can guest rewrite this?

Regards,
Wanpeng Li

> same as not having the CPUID flag, so I'm wondering how this check
> prevents anything harmful ... is it really a cpu "feature"?
>
> If we somehow report ecx bit 1 in CPUID[5], then the guest might try to
> set ecx bit 0 for MWAIT, which will cause #GP(0) and could explain the
> hang that Gabriel is hitting.
>
> Gabriel,
>
>  - do you see VM exits on the "hung" VCPU?
>  - what is your CPU model?
>  - what do you get after running this C program on host and guest?
>
>    #include <stdint.h>
>    #include <stdio.h>
>
>    int main(void) {
>         uint32_t eax = 5, ebx, ecx = 0, edx;
>         asm ("cpuid" : "+a"(eax), "=b"(ebx), "+c"(ecx), "=d"(edx));
>
>         printf("eax=%#08x ebx=%#08x ecx=%#08x edx=%#08x\n", eax, ebx, ecx, edx);
>
>         return 0;
>    }
>
> Thanks.