From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH 2/4] x86: reduce CET-SS related #ifdef-ary
Date: Tue, 28 Jul 2020 21:33:23 +0200 [thread overview]
Message-ID: <d3eb260b-e9e8-1178-828e-73eb119a01ba@suse.com> (raw)
In-Reply-To: <5abaf9e1-c7ba-a58c-d735-47430013eb65@citrix.com>
On 28.07.2020 16:29, Andrew Cooper wrote:
> On 15/07/2020 11:48, Jan Beulich wrote:
>> Now that I've done this I'm not longer sure which direction is better to
>> follow: On one hand this introduces dead code (even if just NOPs) into
>> CET-SS-disabled builds. Otoh this is a step towards breaking the tool
>> chain version dependency of the feature.
>
> The toolchain dependency can't be broken, because of incssp and wrss in C.
>
> There is 0 value and added complexity to trying to partially support
> legacy toolchains.
Complexity: Yes. Zero value - surely not. I'm having a hard time seeing
why you may think so. Would you mind explaining yourself?
> Furthermore, this adds a pile of nops into builds
> which have specifically opted out of CONFIG_XEN_SHSTK, which isn't ideal
> for embedded usecases.
>
> As a consequence, I think its better to keep things consistent with how
> they are now.
>
> One thing I already considered was to make cpu_has_xen_shstk return
> false for !CONFIG_XEN_SHSTK, which subsumes at least one hunk in this
> change.
One is better than nothing, but still pretty little.
>> --- a/xen/arch/x86/x86_64/compat/entry.S
>> +++ b/xen/arch/x86/x86_64/compat/entry.S
>> @@ -198,9 +198,7 @@ ENTRY(cr4_pv32_restore)
>>
>> /* See lstar_enter for entry register state. */
>> ENTRY(cstar_enter)
>> -#ifdef CONFIG_XEN_SHSTK
>> ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
>> -#endif
>
> I can't currently think of any option better than leaving these ifdef's
> in place, other than perhaps
>
> #ifdef CONFIG_XEN_SHSTK
> # define MAYBE_SETSSBSY ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
> #else
> # define MAYBE_SETSSBSY
> #endif
>
> and I don't like it much.
Neither do I. Then we'd also switch STAC/CLAC to MAYBE_STAC / MAYBE_CLAC.
> The think is that everything present there is semantically relevant
> information, and dropping it makes the code worse rather than better.
Everything? I don't see why the #ifdef-s are semantically relevant
(the needed infor is already conveyed by the ALTERNATIVE and its
arguments). I consider them primarily harming readability, and thus I
think we should strive to eliminate them if we can. Hence this patch
...
Jan
next prev parent reply other threads:[~2020-07-28 19:33 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-15 10:47 [PATCH 0/4] x86: some assembler macro rework Jan Beulich
2020-07-15 10:48 ` [PATCH 1/4] x86: replace __ASM_{CL,ST}AC Jan Beulich
2020-07-27 14:55 ` Roger Pau Monné
2020-07-27 19:47 ` Jan Beulich
2020-07-28 9:06 ` Roger Pau Monné
2020-07-31 8:05 ` Jan Beulich
2020-07-31 8:12 ` Roger Pau Monné
2020-07-28 13:59 ` Andrew Cooper
2020-07-28 19:24 ` Jan Beulich
2020-07-28 13:55 ` Andrew Cooper
2020-07-28 19:18 ` Jan Beulich
2020-07-31 8:00 ` Jan Beulich
2020-07-15 10:48 ` [PATCH 2/4] x86: reduce CET-SS related #ifdef-ary Jan Beulich
2020-07-27 15:00 ` Roger Pau Monné
2020-07-27 19:50 ` Jan Beulich
2020-07-28 8:36 ` Roger Pau Monné
2020-07-28 14:29 ` Andrew Cooper
2020-07-28 19:33 ` Jan Beulich [this message]
2020-07-15 10:49 ` [PATCH 3/4] x86: drop ASM_{CL,ST}AC Jan Beulich
2020-07-27 15:10 ` Roger Pau Monné
2020-07-28 14:51 ` Andrew Cooper
2020-07-28 19:41 ` Jan Beulich
2020-07-15 10:49 ` [PATCH 4/4] x86: fold indirect_thunk_asm.h into asm-defns.h Jan Beulich
2020-07-27 15:16 ` Roger Pau Monné
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