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Date:   Fri, 20 Oct 2023 19:21:34 -0700
From:   Pawan Gupta <>
To:     Andrew Cooper <>
Cc:     Thomas Gleixner <>,
        Ingo Molnar <>, Borislav Petkov <>,
        Dave Hansen <>,,
        "H. Peter Anvin" <>,
        Peter Zijlstra <>,
        Josh Poimboeuf <>,
        Andy Lutomirski <>,
        Jonathan Corbet <>,
        Sean Christopherson <>,
        Paolo Bonzini <>,,,,,,,
        Alyssa Milburn <>,
        Daniel Sneddon <>,,
        Alyssa Milburn <>
Subject: Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

On Sat, Oct 21, 2023 at 02:33:47AM +0100, Andrew Cooper wrote:
> On 21/10/2023 2:18 am, Pawan Gupta wrote:
> > On Sat, Oct 21, 2023 at 12:55:45AM +0100, Andrew Cooper wrote:
> >> Also it avoids playing games with hiding data inside an instruction.
> >> It's a neat trick, but the neater trick is avoid it whenever possible.
> > Thanks for the pointers. I think verw in 32-bit mode won't be able to
> > address the operand outside of 4GB range.
> And?  In a 32bit kernel, what lives outside of a 4G range?
> > Maybe this is fine or could it
> > be a problem addressing from e.g. KVM module?
> RIP-relative addressing is disp32.  Which is the same as it is for
> direct calls.
> So if your module is far enough away for VERW to have issues, you've got
> far more basic problems to solve first.

Sorry, I raised the wrong problem. In 64-bit mode, verww only has 32-bit
of relative addressing, so memory operand has to be within 4GB of
callsite. That could be a constraint.

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