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Re: [PATCH v1 8/8] x86: Cleanup cr0.TS flag handling
- To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 23 Mar 2026 13:30:38 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, "Daniel P. Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>, Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 23 Mar 2026 12:30:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19.03.2026 14:29, Ross Lagerwall wrote:
> --- a/xen/arch/x86/cpu/common.c
> +++ b/xen/arch/x86/cpu/common.c
> @@ -883,9 +883,6 @@ void cpu_init(void)
> /* Install correct page table. */
> write_ptbase(current);
>
> - /* Ensure FPU gets initialised for each domain. */
> - stts();
I'm a little concerned by the removal of this and ...
> @@ -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();
>
> if ( cpu_has_xsave )
> fpu_xsave(v);
> else
> fpu_fxsave(v);
> -
> - return true;
> -}
> -
> -void vcpu_save_fpu(struct vcpu *v)
> -{
> - _vcpu_save_fpu(v);
> - stts();
... this. At present it guards us against e.g. an idle CPU or context
switch code mistakenly using in particular XMM registers (but of course
also other extended state).
Jan
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