[PATCH] x86/bhi: avoid hardware mitigation for 'spectre_bhi=vmexit'

[Jon Kohler]

On hardware that supports BHI_DIS_S/X86_FEATURE_BHI_CTRL, do not use
hardware mitigation when using BHI_MITIGATION_VMEXIT_ONLY, as this
causes the value of MSR_IA32_SPEC_CTRL to change, which inflicts
additional KVM overhead.

Example: In a typical eIBRS enabled system, such as Intel SPR, the
SPEC_CTRL may be commonly set to val == 1 to reflect eIBRS enablement;
however, SPEC_CTRL_BHI_DIS_S causes val == 1025. If the guests that
KVM is virtualizing do not also set the guest side value == 1025,
KVM will constantly have to wrmsr toggle the guest vs host value on
both entry and exit, delaying both.

Signed-off-by: Jon Kohler <jon@xxxxxxxxxxx>
---
 arch/x86/kernel/cpu/bugs.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 45675da354f3..df7535f5e882 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1662,8 +1662,16 @@ static void __init bhi_select_mitigation(void)
 			return;
 	}
 
-	/* Mitigate in hardware if supported */
-	if (spec_ctrl_bhi_dis())
+	/*
+	 * Mitigate in hardware if appropriate.
+	 * Note: for vmexit only, do not mitigate in hardware to avoid changing
+	 * the value of MSR_IA32_SPEC_CTRL to include SPEC_CTRL_BHI_DIS_S. If a
+	 * guest does not also set their own SPEC_CTRL to include this, KVM has
+	 * to toggle on every vmexit and vmentry if the host value does not
+	 * match the guest value. Instead, depend on software loop mitigation
+	 * only.
+	 */
+	if (bhi_mitigation != BHI_MITIGATION_VMEXIT_ONLY && spec_ctrl_bhi_dis())
 		return;
 
 	if (!IS_ENABLED(CONFIG_X86_64))
-- 
2.43.0