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Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V
From: |
Vitaly Kuznetsov |
Subject: |
Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure |
Date: |
Wed, 21 Mar 2018 14:18:54 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) |
Roman Kagan <address@hidden> writes:
> On Tue, Mar 20, 2018 at 06:35:00PM +0100, Vitaly Kuznetsov wrote:
>> Requiring tsc_is_stable_and_known() is too restrictive: even without INVTCS
>> nested Hyper-V-on-KVM enables TSC pages for its guests e.g. when
>> Reenlightenment MSRs are present. Presence of frequency MSRs doesn't mean
>> these frequencies are stable, it just means they're available for reading.
>>
>> Signed-off-by: Vitaly Kuznetsov <address@hidden>
>> ---
>> target/i386/kvm.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
>> index 7d9f9ca0b1..74fc3d3b2c 100644
>> --- a/target/i386/kvm.c
>> +++ b/target/i386/kvm.c
>> @@ -651,7 +651,7 @@ static int hyperv_handle_properties(CPUState *cs)
>> env->features[FEAT_HYPERV_EAX] |= HV_TIME_REF_COUNT_AVAILABLE;
>> env->features[FEAT_HYPERV_EAX] |= HV_REFERENCE_TSC_AVAILABLE;
>>
>> - if (has_msr_hv_frequencies && tsc_is_stable_and_known(env)) {
>> + if (has_msr_hv_frequencies && env->tsc_khz) {
>> env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
>> env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
>> }
>
> I suggest that we add a corresponding cpu property here, too. The guest
> may legitimately rely on these msrs when it sees the support in CPUID,
> and migrating from a kernel with the feature supported (4.14+) to an
> older one will make it crash.
>
This can be arranged, but what happens to people who use these features
today? Assuming they also passed 'invtsc' they have stable TSC page
clocksource already (when Hyper-V role is enabled) but when we start
requesting a new 'hv_frequency' cpu property they'll suddenly lose what
they have...
--
Vitaly
- [Qemu-devel] [PATCH v3 0/2] i386/kvm: TSC page clocksource for Hyper-V-on-KVM fixes, Vitaly Kuznetsov, 2018/03/20
- [Qemu-devel] [PATCH v3 1/2] i386/kvm: add support for Hyper-V reenlightenment MSRs, Vitaly Kuznetsov, 2018/03/20
- [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Vitaly Kuznetsov, 2018/03/20
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/21
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure,
Vitaly Kuznetsov <=
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/21
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Eduardo Habkost, 2018/03/21
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/22
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Eduardo Habkost, 2018/03/22
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/22
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Eduardo Habkost, 2018/03/22
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/23
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Eduardo Habkost, 2018/03/23
- Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Roman Kagan, 2018/03/26
Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure, Paolo Bonzini, 2018/03/21