linux-kernel - Re: [PATCH 0/6] IBPB cleanups and a fixup

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Message-ID: <20250220190444.7ytrua37fszvuouy@jpoimboe>
Date: Thu, 20 Feb 2025 11:04:44 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] IBPB cleanups and a fixup

On Wed, Feb 19, 2025 at 10:08:20PM +0000, Yosry Ahmed wrote:
> This series removes X86_FEATURE_USE_IBPB, and fixes a KVM nVMX bug in
> the process. The motivation is mostly the confusing name of
> X86_FEATURE_USE_IBPB, which sounds like it controls IBPBs in general,
> but it only controls IBPBs for spectre_v2_mitigation. A side effect of
> this confusion is the nVMX bug, where virtualizing IBRS correctly
> depends on the spectre_v2_user mitigation.
> 
> The feature bit is mostly redundant, except in controlling the IBPB in
> the vCPU load path. For that, a separate static branch is introduced,
> similar to switch_mm_*_ibpb.

Thanks for doing this.  A few months ago I was working on patches to fix
the same thing but I got preempted multiple times over.

> I wanted to do more, but decided to stay conservative. I was mainly
> hoping to merge indirect_branch_prediction_barrier() with entry_ibpb()
> to have a single IBPB primitive that always stuffs the RSB if the IBPB
> doesn't, but this would add some overhead in paths that currently use
> indirect_branch_prediction_barrier(), and I was not sure if that's
> acceptable.

We always rely on IBPB clearing RSB, so yes, I'd say that's definitely
needed.  In fact I had a patch to do exactly that, with it ending up
like this:

static inline void indirect_branch_prediction_barrier(void)
{
	asm volatile(ALTERNATIVE("", "call write_ibpb", X86_FEATURE_IBPB)
		     : ASM_CALL_CONSTRAINT
		     : : "rax", "rcx", "rdx", "memory");
}

I also renamed "entry_ibpb" -> "write_ibpb" since it's no longer just
for entry code.

-- 
Josh