linux-kernel - [PATCH fsgsbase v2 4/4] x86/fsgsbase: Fix Xen PV support

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Message-Id: <f07c08f178fe9711915862b656722a207cd52c28.1593192140.git.luto@kernel.org>
Date:   Fri, 26 Jun 2020 10:24:30 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org
Subject: [PATCH fsgsbase v2 4/4] x86/fsgsbase: Fix Xen PV support

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@...cle.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Stefano Stabellini <sstabellini@...nel.org>
Cc: xen-devel@...ts.xenproject.org
Signed-off-by: Andy Lutomirski <luto@...nel.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);
+	}
 }
 
 /*
-- 
2.25.4