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

[Jon Kohler]

> On Sep 13, 2024, at 1:39 AM, Chao Gao <chao.gao@xxxxxxxxx> wrote:
> 
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> 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
>> 
>>