[Devel] [PATCH vz9 2/9] ms/x86/virt: Force GIF=1 prior to disabling SVM (for reboot flows)

[Konstantin Khorenko]

From: Maxim Levitsky <mlevitsk at redhat.com>

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2177720

commit 6a3236580b0b1accc3976345e723104f74f6f8e6
Author: Sean Christopherson <seanjc at google.com>
Date:   Wed Nov 30 23:36:48 2022 +0000

    x86/virt: Force GIF=1 prior to disabling SVM (for reboot flows)

    Set GIF=1 prior to disabling SVM to ensure that INIT is recognized if the
    kernel is disabling SVM in an emergency, e.g. if the kernel is about to
    jump into a crash kernel or may reboot without doing a full CPU RESET.
    If GIF is left cleared, the new kernel (or firmware) will be unabled to
    awaken APs.  Eat faults on STGI (due to EFER.SVME=0) as it's possible
    that SVM could be disabled via NMI shootdown between reading EFER.SVME
    and executing STGI.

    Link: https://lore.kernel.org/all/cbcb6f35-e5d7-c1c9-4db9-fe5cc4de579a@amd.com
    Cc: stable at vger.kernel.org
    Cc: Andrew Cooper <Andrew.Cooper3 at citrix.com>
    Cc: Tom Lendacky <thomas.lendacky at amd.com>
    Reviewed-by: Thomas Gleixner <tglx at linutronix.de>
    Link: https://lore.kernel.org/r/20221130233650.1404148-3-seanjc@google.com
    Signed-off-by: Sean Christopherson <seanjc at google.com>

Signed-off-by: Maxim Levitsky <mlevitsk at redhat.com>

(cherry picked from CentOS 9 Stream commit cadcf92788b9)
https://pmc.acronis.work/browse/VSTOR-76102
Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>

Feature: fix ms/KVM
---
 arch/x86/include/asm/virtext.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/virtext.h b/arch/x86/include/asm/virtext.h
index 8757078d4442..3b12e6b99412 100644
--- a/arch/x86/include/asm/virtext.h
+++ b/arch/x86/include/asm/virtext.h
@@ -126,7 +126,21 @@ static inline void cpu_svm_disable(void)
 
 	wrmsrl(MSR_VM_HSAVE_PA, 0);
 	rdmsrl(MSR_EFER, efer);
-	wrmsrl(MSR_EFER, efer & ~EFER_SVME);
+	if (efer & EFER_SVME) {
+		/*
+		 * Force GIF=1 prior to disabling SVM to ensure INIT and NMI
+		 * aren't blocked, e.g. if a fatal error occurred between CLGI
+		 * and STGI.  Note, STGI may #UD if SVM is disabled from NMI
+		 * context between reading EFER and executing STGI.  In that
+		 * case, GIF must already be set, otherwise the NMI would have
+		 * been blocked, so just eat the fault.
+		 */
+		asm_volatile_goto("1: stgi\n\t"
+				  _ASM_EXTABLE(1b, %l[fault])
+				  ::: "memory" : fault);
+fault:
+		wrmsrl(MSR_EFER, efer & ~EFER_SVME);
+	}
 }
 
 /** Makes sure SVM is disabled, if it is supported on the CPU
-- 
2.39.3