Re: [PATCH v3 3/6] x86/bugs: Fix RSB clearing in indirect_branch_prediction_barrier()

[Nikolay Borisov <nik.borisov@suse.com>]

On 2.04.25 г. 21:19 ч., Josh Poimboeuf wrote:
> IBPB is expected to clear the RSB.  However, if X86_BUG_IBPB_NO_RET is
> set, that doesn't happen.  Make indirect_branch_prediction_barrier()
> take that into account by calling __write_ibpb() which already does the
> right thing.

I find this changelog somewhat dubious. So zen < 4 basically have 
IBPB_NO_RET, your patch 2 in this series makes using SBPB for cores 
which have SRSO_NO or if the mitigation is disabled. So if you have a 
core which is zen <4 and doesn't use SBPB then what happens?

> 
> Fixes: 50e4b3b94090 ("x86/entry: Have entry_ibpb() invalidate return predictions")
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
>   arch/x86/include/asm/nospec-branch.h | 6 +++---
>   arch/x86/kernel/cpu/bugs.c           | 1 -
>   2 files changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index bbac79cad04c..f99b32f014ec 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -514,11 +514,11 @@ void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
>   		: "memory");
>   }
>   
> -extern u64 x86_pred_cmd;
> -
>   static inline void indirect_branch_prediction_barrier(void)
>   {
> -	alternative_msr_write(MSR_IA32_PRED_CMD, x86_pred_cmd, X86_FEATURE_IBPB);
> +	asm_inline volatile(ALTERNATIVE("", "call __write_ibpb", X86_FEATURE_IBPB)
> +			    : ASM_CALL_CONSTRAINT
> +			    :: "rax", "rcx", "rdx", "memory");
>   }
>   
>   /* The Intel SPEC CTRL MSR base value cache */
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index c8b8dc829046..9f9637cff7a3 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -59,7 +59,6 @@ DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
>   EXPORT_PER_CPU_SYMBOL_GPL(x86_spec_ctrl_current);
>   
>   u32 x86_pred_cmd __ro_after_init = PRED_CMD_IBPB;
> -EXPORT_SYMBOL_GPL(x86_pred_cmd);
>   
>   static u64 __ro_after_init x86_arch_cap_msr;
>