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

[Chao Gao]

On Thu, Sep 12, 2024 at 07:11:56AM -0700, Jon Kohler wrote:
>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.

Putting aside the security concern, this patch isn't a net positive
because it causes additional overhead to guests with spec_ctrl = 1025.

>
>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
>
>