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

[Juergen Gross <jgross@suse.com>]

On 03.12.24 05:09, Xin Li wrote:
> 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.
> 

We don't.

The wbinvd hook is a leftover from lguest times.

I'll send a patch to remove it.


Juergen

P.S.: As the paravirt maintainer I would have preferred to be Cc-ed in the
       initial patch mail.