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Re: [Xen-devel] [PATCH] x86/spec_ctrl: Fix several bugs in SPEC_CTRL_ENTRY_FROM_INTR_IST

>>> On 14.02.18 at 12:10, <andrew.cooper3@xxxxxxxxxx> wrote:
> DO_OVERWRITE_RSB clobbers %rax, meaning in practice that that the bti_ist_info
> field gets zeroed.  Older versions of this code had the DO_OVERWRITE_RSB
> register selectable, so reintroduce this ability and use it to cause the
> INTR_IST path to use %rdx instead.
> The use of %dl for the %cs.rpl check means that when an IST interrupt hits
> Xen, we try to load 1 into the high 32 bits of MSR_SPEC_CTRL, suffering a #GP
> fault instead.
> Also, drop an unused label which was a copy/paste mistake.
> Reported-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Reported-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one remark/suggestion:

> --- a/xen/include/asm-x86/spec_ctrl_asm.h
> +++ b/xen/include/asm-x86/spec_ctrl_asm.h
> @@ -79,10 +79,10 @@
>   */
> +.macro DO_OVERWRITE_RSB tmp=%rax

If only registers are supposed to be passed as macro arguments,
I generally consider it better to leave specifying at least the % to
the macro body - that way nothing else (like a memory operand,
perhaps referencing %rsp in a dangerous way) can be passed, as
the assembler will choke on invalid names following %. In order to
have the flexibility to also use the 32-bit register name inside the
macro, I would also generally omit the r (but this is less of a concern
here, as it's quite unlikely for the 32-bit name to be needed).



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