> On Sep 13, 2024, at 1:39 AM, Chao Gao <chao.gao@xxxxxxxxx> wrote: > > !-------------------------------------------------------------------| > CAUTION: External Email > > |-------------------------------------------------------------------! > > 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. See my other note with a different approach and testing to match. Responding to this point here that this would require VMM to expose BHI_CTRL which for example QEMU does not, so this is not yet a problem at least today > >> >> 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 >> >>
