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SubjectRe: [PATCH 02/22] x86/srso: Set CPUID feature bits independently of bug or mitigation status
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On 21.08.23 г. 4:18 ч., Josh Poimboeuf wrote:
> Booting with mitigations=off incorrectly prevents the
> X86_FEATURE_{IBPB_BRTYPE,SBPB} CPUID bits from getting set.
>
> Also, future CPUs without X86_BUG_SRSO might still have IBPB with branch
> type prediction flushing, in which case SBPB should be used instead of
> IBPB. The current code doesn't allow for that.
>
> Also, cpu_has_ibpb_brtype_microcode() has some surprising side effects
> and the setting of these feature bits really doesn't belong in the
> mitigation code anyway. Move it to earlier.
>
> Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

LGTM and it's a lot more clear when IBPB_TYPE can actually be set.

Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>


> ---
> arch/x86/include/asm/processor.h | 2 --
> arch/x86/kernel/cpu/amd.c | 28 +++++++++-------------------
> arch/x86/kernel/cpu/bugs.c | 13 +------------
> 3 files changed, 10 insertions(+), 33 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index fd750247ca89..9e26294e415c 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -676,12 +676,10 @@ extern u16 get_llc_id(unsigned int cpu);
> #ifdef CONFIG_CPU_SUP_AMD
> extern u32 amd_get_nodes_per_socket(void);
> extern u32 amd_get_highest_perf(void);
> -extern bool cpu_has_ibpb_brtype_microcode(void);
> extern void amd_clear_divider(void);
> #else
> static inline u32 amd_get_nodes_per_socket(void) { return 0; }
> static inline u32 amd_get_highest_perf(void) { return 0; }
> -static inline bool cpu_has_ibpb_brtype_microcode(void) { return false; }
> static inline void amd_clear_divider(void) { }
> #endif
>
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 7eca6a8abbb1..b08af929135d 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -766,6 +766,15 @@ static void early_init_amd(struct cpuinfo_x86 *c)
>
> if (cpu_has(c, X86_FEATURE_TOPOEXT))
> smp_num_siblings = ((cpuid_ebx(0x8000001e) >> 8) & 0xff) + 1;
> +
> + if (!cpu_has(c, X86_FEATURE_IBPB_BRTYPE)) {

The only way this check can be true is if this kernel is running as a
guest and KVM has synthesized this flag already, right?

> + if (c->x86 == 0x17 && boot_cpu_has(X86_FEATURE_AMD_IBPB))
> + setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE);
> + else if (c->x86 >= 0x19 && !wrmsrl_safe(MSR_IA32_PRED_CMD, PRED_CMD_SBPB)) {
> + setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE);
> + setup_force_cpu_cap(X86_FEATURE_SBPB);
> + }
> + }
> }
>
> static void init_amd_k8(struct cpuinfo_x86 *c)

<snip>

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