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Re: [PATCH] target/i386: properly reset TSC on reset
From: |
Paolo Bonzini |
Subject: |
Re: [PATCH] target/i386: properly reset TSC on reset |
Date: |
Mon, 09 May 2022 23:10:01 +0200 |
As discussed with Marcelo offlist, KVM notices multiple CPUs being set to the
same value within a small amount of time and keeps them synchronized. This is
the same code that handles TSC synchronization on the destination side of
migration.
Paolo
Il 9 maggio 2022 16:54:00 CEST, Marcelo Tosatti <mtosatti@redhat.com> ha
scritto:
>On Thu, Mar 24, 2022 at 06:31:36PM +0100, Paolo Bonzini wrote:
>> Some versions of Windows hang on reboot if their TSC value is greater
>> than 2^54. The calibration of the Hyper-V reference time overflows
>> and fails; as a result the processors' clock sources are out of sync.
>>
>> The issue is that the TSC _should_ be reset to 0 on CPU reset and
>> QEMU tries to do that. However, KVM special cases writing 0 to the
>> TSC and thinks that QEMU is trying to hot-plug a CPU, which is
>> correct the first time through but not later. Thwart this valiant
>> effort and reset the TSC to 1 instead, but only if the CPU has been
>> run once.
>>
>> For this to work, env->tsc has to be moved to the part of CPUArchState
>> that is not zeroed at the beginning of x86_cpu_reset.
>>
>> Reported-by: Vadim Rozenfeld <vrozenfe@redhat.com>
>> Supersedes: <20220324082346.72180-1-pbonzini@redhat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
>Paolo,
>
>Won't this disable the logic to sync TSCs, making it possible
>for TSC of SMP guests to go out of sync? (And remember the logic
>to sync TSCs from within a guest is fragile, in case of VCPU overload
>for example).
>
>> ---
>> target/i386/cpu.c | 13 +++++++++++++
>> target/i386/cpu.h | 2 +-
>> 2 files changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index ec3b50bf6e..cb6b5467d0 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -5931,6 +5931,19 @@ static void x86_cpu_reset(DeviceState *dev)
>> env->xstate_bv = 0;
>>
>> env->pat = 0x0007040600070406ULL;
>> +
>> + if (kvm_enabled()) {
>> + /*
>> + * KVM handles TSC = 0 specially and thinks we are hot-plugging
>> + * a new CPU, use 1 instead to force a reset.
>> + */
>> + if (env->tsc != 0) {
>> + env->tsc = 1;
>> + }
>> + } else {
>> + env->tsc = 0;
>> + }
>> +
>> env->msr_ia32_misc_enable = MSR_IA32_MISC_ENABLE_DEFAULT;
>> if (env->features[FEAT_1_ECX] & CPUID_EXT_MONITOR) {
>> env->msr_ia32_misc_enable |= MSR_IA32_MISC_ENABLE_MWAIT;
>> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
>> index e31e6bd8b8..982c532353 100644
>> --- a/target/i386/cpu.h
>> +++ b/target/i386/cpu.h
>> @@ -1554,7 +1554,6 @@ typedef struct CPUArchState {
>> target_ulong kernelgsbase;
>> #endif
>>
>> - uint64_t tsc;
>> uint64_t tsc_adjust;
>> uint64_t tsc_deadline;
>> uint64_t tsc_aux;
>> @@ -1708,6 +1707,7 @@ typedef struct CPUArchState {
>> int64_t tsc_khz;
>> int64_t user_tsc_khz; /* for sanity check only */
>> uint64_t apic_bus_freq;
>> + uint64_t tsc;
>> #if defined(CONFIG_KVM) || defined(CONFIG_HVF)
>> void *xsave_buf;
>> uint32_t xsave_buf_len;
>> --
>> 2.35.1
>>
>>
>>
>