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Re: [PATCH v1 2/8] x86/vmx: Remove lazy FPU support
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 19 Mar 2026 17:43:34 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 19 Mar 2026 16:43:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19.03.2026 17:38, Andrew Cooper wrote:
> On 19/03/2026 1:29 pm, Ross Lagerwall wrote:
>> Remove lazy FPU support from the VMX code since fully_eager_fpu is now
>> always true.
>>
>> No functional change intended.
>>
>> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
>> ---
>> xen/arch/x86/hvm/vmx/vmcs.c | 8 +--
>> xen/arch/x86/hvm/vmx/vmx.c | 70 +------------------------
>> xen/arch/x86/hvm/vmx/vvmx.c | 15 +-----
>> xen/arch/x86/include/asm/hvm/vmx/vmcs.h | 2 -
>> 4 files changed, 5 insertions(+), 90 deletions(-)
>>
>> diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
>> index c2e7f9aed39f..8e52ef4d497a 100644
>> --- a/xen/arch/x86/hvm/vmx/vmcs.c
>> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
>> @@ -1247,10 +1247,7 @@ static int construct_vmcs(struct vcpu *v)
>> __vmwrite(HOST_TR_SELECTOR, TSS_SELECTOR);
>>
>> /* Host control registers. */
>> - v->arch.hvm.vmx.host_cr0 = read_cr0() & ~X86_CR0_TS;
>> - if ( !v->arch.fully_eager_fpu )
>> - v->arch.hvm.vmx.host_cr0 |= X86_CR0_TS;
>> - __vmwrite(HOST_CR0, v->arch.hvm.vmx.host_cr0);
>> + __vmwrite(HOST_CR0, read_cr0());
>
> (Not for this patch) but I'm pretty sure there's room to optimise this
> further.
>
> CR0 should be constant, both here and in SVM. Reading the active cr0 is
> an example of the anti-pattern we need to purge to make nested-virt work
> better.
In which case, is it a good idea to purge the host_cr0 field?
Jan
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