Re: [PATCH fsgsbase v2 4/4] x86/fsgsbase: Fix Xen PV support

[Jürgen Groß <jgross@xxxxxxxx>]

On 26.06.20 19:24, Andy Lutomirski wrote:
> On Xen PV, SWAPGS doesn't work.  Teach __rdfsbase_inactive() and
> __wrgsbase_inactive() to use rdmsrl()/wrmsrl() on Xen PV.  The Xen
> pvop code will understand this and issue the correct hypercalls.
>
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>   arch/x86/kernel/process_64.c | 20 ++++++++++++++------
>   1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index cb8e37d3acaa..457d02aa10d8 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -163,9 +163,13 @@ static noinstr unsigned long __rdgsbase_inactive(void)
>   
>   	lockdep_assert_irqs_disabled();
>   
> -	native_swapgs();
> -       gsbase = rdgsbase();
> -       native_swapgs();
> +       if (!static_cpu_has(X86_FEATURE_XENPV)) {
> +               native_swapgs();
> +               gsbase = rdgsbase();
> +               native_swapgs();
> +       } else {
> +               rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
> +       }
>   
>   	return gsbase;
>   }
> @@ -182,9 +186,13 @@ static noinstr void __wrgsbase_inactive(unsigned long 
> gsbase)
>   {
>         lockdep_assert_irqs_disabled();
>   
> -	native_swapgs();
> -       wrgsbase(gsbase);
> -       native_swapgs();
> +       if (!static_cpu_has(X86_FEATURE_XENPV)) {
> +               native_swapgs();
> +               wrgsbase(gsbase);
> +               native_swapgs();
> +       } else {
> +               wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
> +       }
>   }
>   
>   /*
Another possibility would be to just do (I'm fine either way):

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index acc49fa6a097..b78dd373adbf 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -318,6 +318,8 @@ static void __init xen_init_capabilities(void)
                setup_clear_cpu_cap(X86_FEATURE_XSAVE);
                setup_clear_cpu_cap(X86_FEATURE_OSXSAVE);
        }
+
+       setup_clear_cpu_cap(X86_FEATURE_FSGSBASE);
 }

 static void xen_set_debugreg(int reg, unsigned long val)


Juergen