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Subject: Re: [v3 1/1] x86/cpufeatures: Implement Predictive Store Forwarding
 control.
To:     Ramakrishna Saripalli <rsaripal@amd.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        Jonathan Corbet <corbet@lwn.net>
Cc:     bsd@redhat.com
References: <20210428160349.158774-1-rsaripal@amd.com>
 <20210428160349.158774-2-rsaripal@amd.com>
From:   Tom Lendacky <thomas.lendacky@amd.com>
Message-ID: <b8135c13-591c-12b0-d5fb-900e146ffbad@amd.com>
Date:   Thu, 29 Apr 2021 00:13:14 -0500
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On 4/28/21 11:03 AM, Ramakrishna Saripalli wrote:
> From: Ramakrishna Saripalli <rk.saripalli@amd.com>
> 
> Certain AMD processors feature a new technology called Predictive Store
> Forwarding (PSF).
> 
> PSF is a micro-architectural optimization designed to improve the
> performance of code execution by predicting dependencies between
> loads and stores.
> 
> Incorrect PSF predictions can occur due to two reasons.
> 
> - It is possible that the load/store pair may have had dependency for
>   a while but the dependency has stopped because the address in the
>   load/store pair has changed.
> 
> - Second source of incorrect PSF prediction can occur because of an alias
>   in the PSF predictor structure stored in the microarchitectural state.
>   PSF predictor tracks load/store pair based on portions of instruction
>   pointer. It is possible that a load/store pair which does have a
>   dependency may be aliased by another load/store pair which does not have
>   the same dependency. This can result in incorrect speculation.
> 
>   Software may be able to detect this aliasing and perform side-channel
>   attacks.
> 
> All CPUs that implement PSF provide one bit to disable this feature.
> If the bit to disable this feature is available, it means that the CPU
> implements PSF feature and is therefore vulnerable to PSF risks.
> 
> The bits that are introduced
> 
> X86_FEATURE_PSFD: CPUID_Fn80000008_EBX[28] ("PSF disable")
> 	If this bit is 1, CPU implements PSF and PSF control
> 	via SPEC_CTRL_MSR is supported in the CPU.
> 
> All AMD processors that support PSF implement a bit in
> SPEC_CTRL MSR (0x48) to disable or enable Predictive Store
> Forwarding.
> 
> PSF control introduces a new kernel parameter called
> 	predict_store_fwd.
> 
> Kernel parameter predict_store_fwd has the following values
> 
> - off. This value is used to disable PSF on all CPUs.
> 
> - on. This value is used to enable PSF on all CPUs.
>         This is also the default setting.
> ---
> ChangeLogs:
>     V3->V2:
>           Set the X86_FEATURE_SPEC_CTRL_MSR cap in boot cpu caps.
>           Fix kernel documentation for the kernel parameter.
>           Rename PSF to a control instead of mitigation.
> 
>     V1->V2:
> 	- Smashed multiple commits into one commit.
> 	- Rename PSF to a control instead of mitigation.
> 
>     V1:
> 	- Initial patchset.
> 	- Kernel parameter controls enable and disable of PSF.
> ====================
> Signed-off-by: Ramakrishna Saripalli<rk.saripalli@amd.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  5 +++++
>  arch/x86/include/asm/cpufeatures.h            |  1 +
>  arch/x86/include/asm/msr-index.h              |  2 ++
>  arch/x86/kernel/cpu/amd.c                     | 20 +++++++++++++++++++
>  4 files changed, 28 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index de27d5a4d994..0576e8a8d033 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3950,6 +3950,11 @@
>  			Format: {"off"}
>  			Disable Hardware Transactional Memory
>  
> +	predict_store_fwd=	[X86] This option controls PSF.
> +			off - Turns off PSF.
> +			on  - Turns on PSF.
> +			default : on.
> +
>  	preempt=	[KNL]
>  			Select preemption mode if you have CONFIG_PREEMPT_DYNAMIC
>  			none - Limited to cond_resched() calls
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 3c94316169a3..e36e6bf2f18b 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -313,6 +313,7 @@
>  #define X86_FEATURE_AMD_SSBD		(13*32+24) /* "" Speculative Store Bypass Disable */
>  #define X86_FEATURE_VIRT_SSBD		(13*32+25) /* Virtualized Speculative Store Bypass Disable */
>  #define X86_FEATURE_AMD_SSB_NO		(13*32+26) /* "" Speculative Store Bypass is fixed in hardware. */
> +#define X86_FEATURE_PSFD		(13*32+28) /* Predictive Store Forward Disable */
>  
>  /* Thermal and Power Management Leaf, CPUID level 0x00000006 (EAX), word 14 */
>  #define X86_FEATURE_DTHERM		(14*32+ 0) /* Digital Thermal Sensor */
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 742d89a00721..21f0c3fc1b2c 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -51,6 +51,8 @@
>  #define SPEC_CTRL_STIBP			BIT(SPEC_CTRL_STIBP_SHIFT)	/* STIBP mask */
>  #define SPEC_CTRL_SSBD_SHIFT		2	   /* Speculative Store Bypass Disable bit */
>  #define SPEC_CTRL_SSBD			BIT(SPEC_CTRL_SSBD_SHIFT)	/* Speculative Store Bypass Disable */
> +#define SPEC_CTRL_PSFD_SHIFT		7
> +#define SPEC_CTRL_PSFD			BIT(SPEC_CTRL_PSFD_SHIFT)	/* Predictive Store Forwarding Disable */
>  
>  #define MSR_IA32_PRED_CMD		0x00000049 /* Prediction Command */
>  #define PRED_CMD_IBPB			BIT(0)	   /* Indirect Branch Prediction Barrier */
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 2d11384dc9ab..c9b6ba3ea431 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -1165,3 +1165,23 @@ void set_dr_addr_mask(unsigned long mask, int dr)
>  		break;
>  	}
>  }
> +
> +static int __init psf_cmdline(char *str)
> +{
> +	if (!boot_cpu_has(X86_FEATURE_PSFD))
> +		return 0;
> +
> +	if (!str)
> +		return -EINVAL;
> +
> +	if (!strcmp(str, "off")) {
> +		set_cpu_cap(&boot_cpu_data, X86_FEATURE_MSR_SPEC_CTRL);
> +		x86_spec_ctrl_base |= SPEC_CTRL_PSFD;
> +		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> +		setup_clear_cpu_cap(X86_FEATURE_PSFD);

Why are you clearing the feature here? Won't this be needed for
virtualization support?

Thanks,
Tom

> +	}
> +
> +	return 0;
> +}
> +
> +early_param("predict_store_fwd", psf_cmdline);
>