From: Dave Hansen <dave.hansen@intel.com>
To: Ingo Molnar <mingo@kernel.org>, Ira Weiny <ira.weiny@intel.com>,
Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>, Rik van Riel <riel@surriel.com>,
x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry
Date: Mon, 8 Aug 2022 09:16:43 -0700 [thread overview]
Message-ID: <01768c70-a012-0d08-cfa5-a7a87ff0a19c@intel.com> (raw)
In-Reply-To: <YvDtfKRyMGenRMU5@gmail.com>
On 8/8/22 04:03, Ingo Molnar wrote:
>> Again, I don't believe this is too much overhead but I don't want people
>> to say it was not discussed.
> Is it necessary to do this, what are the alternatives, can this overhead be
> avoided?
One thing Andy mentioned is that we _could_ get it down to two instructions:
rdgsbase $reg
push $reg
This could be hidden in:
PUSH_PTREGS_AUXILIARY
where, today, it would only net add a single instruction. But, if we
ever add more stuff to PUSH_PTREGS_AUXILIARY, it would move back to
needing two instructions since we'd need both the:
subq $PTREGS_AUX_SIZE, %rsp
and something to write gsbase to the stack.
That doesn't get us the smp_processor_id() directly, but we can derive
it later on from the gsbase value.
The downside is that we're doing it in assembly. We'd also have
something additional which is a bit uglier and that reads memory on
!X86_FEATURE_FSGSBASE, probably:
movq PER_CPU_VAR(cpu_number), %reg
push %reg
Which would require some different code to decode what was there:
int read_exception_cpu_number(ext_pt_regs *e)
{
if (cpu_feature_enabled(X86_FEATURE_FSGSBASE))
return gsbase_to_cpu_number(e->ext_cpu_nr);
else
return e->ext_cpu_nr;
}
I'm thinking that the whole racy smp_processor_id() thing wasn't so bad
in the first place.
next prev parent reply other threads:[~2022-08-08 16:16 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-05 17:30 [RFC PATCH 0/5] Print CPU at segfault time ira.weiny
2022-08-05 17:30 ` [RFC PATCH 1/5] entry: Pass pt_regs to irqentry_exit_cond_resched() ira.weiny
2022-08-05 18:33 ` Rik van Riel
2022-08-08 10:38 ` Borislav Petkov
2022-08-08 17:34 ` Ira Weiny
2022-08-08 17:38 ` Borislav Petkov
2022-08-08 17:43 ` Dave Hansen
2022-08-08 17:52 ` Borislav Petkov
2022-08-09 23:18 ` Thomas Gleixner
2022-08-10 7:25 ` Thomas Gleixner
2022-08-05 17:30 ` [RFC PATCH 2/5] entry: Add calls for save/restore auxiliary pt_regs ira.weiny
2022-08-05 18:34 ` Rik van Riel
2022-08-09 12:05 ` Borislav Petkov
2022-08-09 18:38 ` Ira Weiny
2022-08-09 18:49 ` Borislav Petkov
2022-08-09 21:14 ` Thomas Gleixner
2022-08-09 21:38 ` Borislav Petkov
2022-08-09 22:33 ` Ira Weiny
2022-08-09 21:49 ` Ira Weiny
2022-08-09 23:53 ` Thomas Gleixner
2022-08-05 17:30 ` [RFC PATCH 3/5] x86/entry: Add auxiliary pt_regs space ira.weiny
2022-08-05 18:45 ` Rik van Riel
2022-08-05 17:30 ` [RFC PATCH 4/5] x86,mm: print likely CPU at segfault time ira.weiny
2022-08-05 17:30 ` [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry ira.weiny
2022-08-05 18:46 ` Rik van Riel
2022-08-05 18:47 ` Dave Hansen
2022-08-06 9:01 ` Ingo Molnar
2022-08-06 9:11 ` Borislav Petkov
2022-08-07 10:02 ` Ingo Molnar
2022-08-07 10:35 ` Borislav Petkov
2022-08-07 20:02 ` Ira Weiny
2022-08-08 11:03 ` Ingo Molnar
2022-08-08 12:01 ` Borislav Petkov
2022-08-09 20:06 ` Thomas Gleixner
2022-08-08 16:16 ` Dave Hansen [this message]
2022-08-08 17:24 ` Rik van Riel
2022-08-09 21:19 ` Andy Lutomirski
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