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    SubjectRe: [PATCH 1/6] x86/bugs: Add asm helpers for executing VERW
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    On Fri, Oct 20, 2023, Pawan Gupta wrote:
    > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
    > index c55cc243592e..e1b623a27e1b 100644
    > --- a/arch/x86/include/asm/nospec-branch.h
    > +++ b/arch/x86/include/asm/nospec-branch.h
    > @@ -111,6 +111,24 @@
    > #define RESET_CALL_DEPTH_FROM_CALL
    > #endif
    >
    > +/*
    > + * Macro to execute VERW instruction to mitigate transient data sampling
    > + * attacks such as MDS. On affected systems a microcode update overloaded VERW
    > + * instruction to also clear the CPU buffers.
    > + *
    > + * Note: Only the memory operand variant of VERW clears the CPU buffers. To
    > + * handle the case when VERW is executed after user registers are restored, use
    > + * RIP to point the memory operand to a part NOPL instruction that contains
    > + * __KERNEL_DS.
    > + */
    > +#define __EXEC_VERW(m) verw _ASM_RIP(m)
    > +
    > +#define EXEC_VERW \
    > + __EXEC_VERW(551f); \
    > + /* nopl __KERNEL_DS(%rax) */ \
    > + .byte 0x0f, 0x1f, 0x80, 0x00, 0x00; \
    > +551: .word __KERNEL_DS; \

    Why are there so many macro layers? Nothing jumps out to justfying two layers,
    let alone three.

    > +
    > /*
    > * Fill the CPU return stack buffer.
    > *
    > @@ -329,6 +347,13 @@
    > #endif
    > .endm
    >
    > +/* Clear CPU buffers before returning to user */
    > +.macro USER_CLEAR_CPU_BUFFERS
    > + ALTERNATIVE "jmp .Lskip_verw_\@;", "", X86_FEATURE_USER_CLEAR_CPU_BUF
    > + EXEC_VERW

    Rather than a NOP after VERW, why not something like this?

    /* Clear CPU buffers before returning to user */
    .macro USER_CLEAR_CPU_BUFFERS
    ALTERNATIVE "jmp .Lskip_verw_\@;", "jmp .Ldo_verw_\@;", X86_FEATURE_USER_CLEAR_CPU_BUF
    551: .word __KERNEL_DS
    .Ldo_verw_\@: verw _ASM_RIP(551b)
    .Lskip_verw_\@:
    .endm

    > +.Lskip_verw_\@:
    > +.endm

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