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Re: [PATCH] x86/spec-ctrl: Build with BRANCH_HARDEN lfences by default

  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 7 Oct 2021 10:37:12 +0200
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  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Thu, 07 Oct 2021 08:37:20 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 05.10.2021 13:11, Andrew Cooper wrote:
> Branch Harden is enabled by default at compile and boot time.  Invert the code
> to compile with lfence by default and nop out in the non-default case.
> This has several advantages.  It removes 3829 patch points (in the random
> build of Xen I have to hand) by default on boot, 70% (!) of the
> .altinstr_replacement section.  For builds of Xen with a non-nops capable tool
> chain, the code after `spec-ctrl=no-branch-harden` is better because Xen can
> write long nops.
> Most importantly however, it means the disassembly actually matches what runs
> in the common case, with the ability to distinguish the lfences from other
> uses of nops.
> Finally, make opt_branch_harden local to spec_ctrl.c and __initdata.  It has
> never been used externally, even at it's introduction in c/s 3860d5534df4
> "spec: add l1tf-barrier".
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>



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