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

[Pawan Gupta]

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.
> 
> 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;

This makes the system vulnerable. The current software loop is not
effective on parts that support BHI_DIS_S. There is a separate loop for
SPR, see Listing 2(long sequence) in Software BHB-clearing sequence
section here:

  https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/branch-history-injection.html

It is only worth implementing the long sequence in VMEXIT_ONLY mode if it is
significantly better than toggling the MSR.