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Subject: Re: [PATCH fsgsbase v2 4/4] x86/fsgsbase: Fix Xen PV support
To:     Andy Lutomirski <luto@kernel.org>, x86@kernel.org
Cc:     Sasha Levin <sashal@kernel.org>, linux-kernel@vger.kernel.org,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        xen-devel@lists.xenproject.org
References: <cover.1593192140.git.luto@kernel.org>
 <f07c08f178fe9711915862b656722a207cd52c28.1593192140.git.luto@kernel.org>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <714d4c04-5907-885f-e23b-baef662f1080@suse.com>
Date:   Mon, 29 Jun 2020 07:17:20 +0200
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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@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: xen-devel@lists.xenproject.org
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>   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