Re: [RFC PATCH 1/2] x86, lib, xenpv: Add WBNOINVD helper functions

Xin Li
2024-12-03T04:10:46Z

On 12/2/2024 5:44 PM, Kevin Loughlin wrote:
> On Mon, Dec 2, 2024 at 5:28 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>
>> On 03/12/2024 12:59 am, Kevin Loughlin wrote:
>>> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
>>> index d4eb9e1d61b8..c040af2d8eff 100644
>>> --- a/arch/x86/include/asm/paravirt.h
>>> +++ b/arch/x86/include/asm/paravirt.h
>>> @@ -187,6 +187,13 @@ static __always_inline void wbinvd(void)
>>> PVOP_ALT_VCALL0(cpu.wbinvd, "wbinvd", ALT_NOT_XEN);
>>> }
>>>
>>> +extern noinstr void pv_native_wbnoinvd(void);
>>> +
>>> +static __always_inline void wbnoinvd(void)
>>> +{
>>> + PVOP_ALT_VCALL0(cpu.wbnoinvd, "wbnoinvd", ALT_NOT_XEN);
>>> +}
>>
>> Given this, ...
>>
>>> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
>>> index fec381533555..a66b708d8a1e 100644
>>> --- a/arch/x86/kernel/paravirt.c
>>> +++ b/arch/x86/kernel/paravirt.c
>>> @@ -149,6 +154,7 @@ struct paravirt_patch_template pv_ops = {
>>> .cpu.write_cr0 = native_write_cr0,
>>> .cpu.write_cr4 = native_write_cr4,
>>> .cpu.wbinvd = pv_native_wbinvd,
>>> + .cpu.wbnoinvd = pv_native_wbnoinvd,
>>> .cpu.read_msr = native_read_msr,
>>> .cpu.write_msr = native_write_msr,
>>> .cpu.read_msr_safe = native_read_msr_safe,
>>
>> this, and ...
>>
>>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
>>> index d6818c6cafda..a5c76a6f8976 100644
>>> --- a/arch/x86/xen/enlighten_pv.c
>>> +++ b/arch/x86/xen/enlighten_pv.c
>>> @@ -1162,6 +1162,7 @@ static const typeof(pv_ops) xen_cpu_ops __initconst = {
>>> .write_cr4 = xen_write_cr4,
>>>
>>> .wbinvd = pv_native_wbinvd,
>>> + .wbnoinvd = pv_native_wbnoinvd,
>>>
>>> .read_msr = xen_read_msr,
>>> .write_msr = xen_write_msr,
>>
>> this, what is the point having a paravirt hook which is wired to
>> native_wbnoinvd() in all cases?
>>
>> That just seems like overhead for overhead sake.
>
> I'm mirroring what's done for WBINVD here, which was changed to a
> paravirt hook in 10a099405fdf ("cpuidle, xenpv: Make more PARAVIRT_XXL
> noinstr clean") in order to avoid calls out to instrumented code as
> described in the commit message in more detail. I believe a hook is
> similarly required for WBNOINVD, but please let me know if you
> disagree. Thanks!

Then the question is why we need to add WBINVD/WBNOINVD to the paravirt
hooks.