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 > >
