Re: [PATCH] target/i386: properly reset TSC on reset

[Marcelo Tosatti <mtosatti@redhat.com>]

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
> 
> 
>