linux-kernel - Re: [PATCH 26/26] KVM: x86: asi: Add some mitigations on address space transitions

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Message-ID: <CA+i-1C1pdcdk0LYBJ=3mMqmP6x91Wpbo64ekV3uUirLpsJtjJQ@mail.gmail.com>
Date: Wed, 21 Aug 2024 11:38:21 +0200
From: Brendan Jackman <jackmanb@...gle.com>
To: Shivank Garg <shivankg@....com>
Cc: 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>, 
	Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Alexandre Chartre <alexandre.chartre@...cle.com>, Liran Alon <liran.alon@...cle.com>, 
	Jan Setje-Eilers <jan.setjeeilers@...cle.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Mel Gorman <mgorman@...e.de>, 
	Lorenzo Stoakes <lstoakes@...il.com>, David Hildenbrand <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Michal Hocko <mhocko@...nel.org>, Khalid Aziz <khalid.aziz@...cle.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Valentin Schneider <vschneid@...hat.com>, Paul Turner <pjt@...gle.com>, Reiji Watanabe <reijiw@...gle.com>, 
	Junaid Shahid <junaids@...gle.com>, Ofir Weisse <oweisse@...gle.com>, 
	Yosry Ahmed <yosryahmed@...gle.com>, Patrick Bellasi <derkling@...gle.com>, 
	KP Singh <kpsingh@...gle.com>, Alexandra Sandulescu <aesa@...gle.com>, 
	Matteo Rizzo <matteorizzo@...gle.com>, Jann Horn <jannh@...gle.com>, x86@...nel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, kvm@...r.kernel.org
Subject: Re: [PATCH 26/26] KVM: x86: asi: Add some mitigations on address
 space transitions

Hi Shivank,

On Tue, 20 Aug 2024 at 11:52, Shivank Garg <shivankg@....com> wrote:
>
>  .pushsection .noinstr.text, "ax"
>  SYM_CODE_START(fill_return_buffer)
> +       UNWIND_HINT_FUNC
>         __FILL_RETURN_BUFFER(%_ASM_AX,RSB_CLEAR_LOOPS)
>         RET
>  SYM_CODE_END(fill_return_buffer)
> +__EXPORT_THUNK(fill_return_buffer)
>  .popsection

Thanks a lot for the pointer! UNWIND_HINT_FUNC does indeed seem to be
what I was missing with the objtool warning.

Regarding the build failure, could you share your config/toolchain
info so I can try to reproduce? Would be handy for checking my next
posting. Now I see your mail, it seems surprising that it compiles for
me.

Also while I'm replying to this thread I'll note this:

> +       if (!IS_ENABLED(CONFIG_RETPOLINE) ||
> +           !cpu_feature_enabled(X86_FEATURE_RSB_VMEXIT))

It's called CONFIG_MITIGATION_RETPOLINE now.

And furthermore, kvm_get_running_vcpu needs to be noinstr, I'm getting
an objtool warning about this that wasn't mentioned in my cover
letter.