Re: [PATCH 11/13] vmx: Use scaled host TSC to calculate TSC offset

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 09/28/2015 03:13 AM, Haozhong Zhang wrote:
> If VMX TSC scaling is enabled and no TSC emulation is used,
> vmx_set_tsc_offset() will calculate the TSC offset by substracting the
> scaled host TSC from the current guest TSC.
>
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> ---
>   xen/arch/x86/hvm/vmx/vmx.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
>
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index 454440e..163974d 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -1102,11 +1102,26 @@ static void vmx_handle_cd(struct vcpu *v, unsigned long value)
>   
>   static void vmx_set_tsc_offset(struct vcpu *v, u64 offset, u64 at_tsc)
>   {
> +    uint64_t host_tsc, guest_tsc;
> +    struct domain *d = v->domain;
> +
> +    guest_tsc = hvm_get_guest_tsc_fixed(v, at_tsc);
> +
> +    if ( cpu_has_vmx_tsc_scaling && !d->arch.vtsc )
> +    {
> +        host_tsc = at_tsc ? at_tsc : rdtsc();
> +        offset = guest_tsc - hvm_scale_tsc(v, host_tsc);
> +    }
> +
>       vmx_vmcs_enter(v);
>   
> +    if ( !nestedhvm_enabled(d) )
> +        goto out;
> +
>       if ( nestedhvm_vcpu_in_guestmode(v) )
>           offset += nvmx_get_tsc_offset(v);
>   
> +out:
>       __vmwrite(TSC_OFFSET, offset);
>       vmx_vmcs_exit(v);
>   }


This (and corresponding SVM code) looks somewhat suspect to me: if the 
processor supports scaling we are ignoring caller-provided offset.

Besides, at least when called from hvm_set_guest_tsc_fixed() --- we've 
already taken scaling into account, that's what patch 6 does, doesn't it?


-boris