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Re: [PATCH v1 8/8] x86: Cleanup cr0.TS flag handling
- To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 19 Mar 2026 20:53:40 +0000
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, "Daniel P. Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>, Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 19 Mar 2026 20:54:00 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19/03/2026 1:29 pm, Ross Lagerwall wrote:
> diff --git a/xen/arch/x86/i387.c b/xen/arch/x86/i387.c
> index 9acaaf4673df..336bc83b6e13 100644
> --- a/xen/arch/x86/i387.c
> +++ b/xen/arch/x86/i387.c
> @@ -193,31 +190,18 @@ void vcpu_restore_fpu(struct vcpu *v)
> * On each context switch, save the necessary FPU info of VCPU being switch
> * out. It dispatches saving operation based on CPU's capability.
> */
> -static bool _vcpu_save_fpu(struct vcpu *v)
> +void vcpu_save_fpu(struct vcpu *v)
> {
> ASSERT(!is_idle_vcpu(v));
>
> /* This can happen, if a paravirtualised guest OS has set its CR0.TS. */
> - clts();
> + if ( is_pv_vcpu(v) )
> + clts();
>
It's quite likely that this would be quicker to just leave as unconditional.
is_pv_vcpu() has evaluate_nospec() in it, so forces LFENCEs, and CLTS
has a fast nop path even in very early implementations.
~Andrew
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