From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH] x86/boot: Simplify %fs setup in trampoline_setup
Date: Tue, 27 Aug 2019 16:24:30 +0200 [thread overview]
Message-ID: <13ea9a6d-733c-42ce-4bd1-e73cc7b2cba1@suse.com> (raw)
In-Reply-To: <20190812151032.9353-1-andrew.cooper3@citrix.com>
On 12.08.2019 17:10, Andrew Cooper wrote:
> mov/shr is easier to follow than shld, and doesn't have a merge dependency on
> the previous value of %edx. Shorten the rest of the code by streamlining the
> comments.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Wei Liu <wl@xen.org>
> CC: Roger Pau Monné <roger.pau@citrix.com>
>
> In addition to being clearer to follow, mov/shr is faster than shld to decode
> and execute. See https://godbolt.org/z/A5kvuC for the latency/throughput/port
> analysis, the Intel Optimisation guide which classifes them as "Slow Int"
> instructions, or the AMD Optimisation guide which specifically has a section
> entitled "Alternatives to SHLD Instruction".
I don't really mind the change, but I don't think performance is a
concern here. Instead I think we want to size-optimize the trampoline
as much as possible, which is why (iirc) I had asked for the use of
SHLD here. Considering David's work to split boot and permanent
trampoline I'm find with the minimal 1 byte increase though:
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Jan
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prev parent reply other threads:[~2019-08-27 14:24 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-12 15:10 [Xen-devel] [PATCH] x86/boot: Simplify %fs setup in trampoline_setup Andrew Cooper
2019-08-13 11:05 ` Wei Liu
2019-08-27 14:24 ` Jan Beulich [this message]
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