From: Juergen Gross <jgross@suse.com>
To: Lai Jiangshan <jiangshanlai@gmail.com>, Borislav Petkov <bp@alien8.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
Lai Jiangshan <jiangshan.ljs@antgroup.com>,
Ingo Molnar <mingo@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH V6 3/8] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()
Date: Mon, 2 May 2022 14:42:50 +0200 [thread overview]
Message-ID: <6c0dd7c7-593b-d8cf-347b-3d32310fcc11@suse.com> (raw)
In-Reply-To: <CAJhGHyANWhu-HX20_4XhgACE8tUd8SFa5BTH-ynJscNQ7QxBRw@mail.gmail.com>
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On 28.04.22 02:33, Lai Jiangshan wrote:
> On Thu, Apr 28, 2022 at 1:45 AM Borislav Petkov <bp@alien8.de> wrote:
>>
>> On Thu, Apr 21, 2022 at 10:10:50PM +0800, Lai Jiangshan wrote:
>>> From: Lai Jiangshan <jiangshan.ljs@antgroup.com>
>>>
>>> The macro idtentry calls error_entry() unconditionally even on XENPV.
>>> But the code XENPV needs in error_entry() is PUSH_AND_CLEAR_REGS only.
>>> And error_entry() also calls sync_regs() which has to deal with the
>>> case of XENPV via an extra branch so that it doesn't copy the pt_regs.
>>
>> What extra branch?
>>
>> Do you mean the
>>
>> if (regs != eregs)
>>
>> test in sync_regs()?
>
> Hello, Borislav
>
> Yes.
>
>>
>> I'm confused. Are you, per chance, aiming to optimize XENPV here or
>> what's up?
>
>
> The branch in sync_regs() can be optimized out for the non-XENPV case
> since XENPV doesn't call sync_regs() after patch5 which makes XENPV
> not call error_entry().
>
> The aim of this patch and most of the patchset is to make
> error_entry() be able to be converted to C. And XENPV cases can
> also be optimized in the patchset although it is not the major main.
>
>>
>>> And PUSH_AND_CLEAR_REGS in error_entry() makes the stack not return to
>>> its original place when the function returns, which means it is not
>>> possible to convert it to a C function.
>>>
>>> Move PUSH_AND_CLEAR_REGS out of error_entry(), add a function to wrap
>>> PUSH_AND_CLEAR_REGS and call it before error_entry().
>>>
>>> The new function call adds two instructions (CALL and RET) for every
>>> interrupt or exception.
>>
>> Not only - it pushes all the regs in PUSH_AND_CLEAR_REGS too. I don't
>> understand why that matters here? It was done in error_entry anyway.
>>
>
> Compared to the original code, adding the new function call adds two
> instructions (CALL and RET) for every interrupt or exception.
>
> PUSH_AND_CLEAR_REGS is not extra instructions added here.
>
> Since this patch adds extra overhead (CALL and RET), the changelog
> has to explain why it is worth it not just for converting ASM to C.
>
> The explanation in the changelog is that it can be offsetted by later
> reduced overhead.
I think you could avoid the extra call/ret by doing something like:
SYM_CODE_START_LOCAL(error_exit_push_and_save)
UNWIND_HINT_FUNC
PUSH_AND_CLEAR_REGS save_ret=1
ENCODE_FRAME_POINTER 8
jmp error_exit
SYM_CODE_END(error_exit_push_and_save)
... and use this instead of patch 5:
ALTERNATIVE "call error_entry_push_and_save; movq %rax, %rsp", \
"call push_and_clear_regs", X86_FEATURE_XENPV
Juergen
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next prev parent reply other threads:[~2022-05-02 12:43 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-21 14:10 [PATCH V6 0/8] x86/entry: Clean up entry code Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 1/8] x86/traps: Move pt_regs only in fixup_bad_iret() Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 2/8] x86/entry: Switch the stack after error_entry() returns Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 3/8] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry() Lai Jiangshan
2022-04-27 17:45 ` Borislav Petkov
2022-04-28 0:33 ` Lai Jiangshan
2022-04-28 10:26 ` Borislav Petkov
2022-05-02 12:42 ` Juergen Gross [this message]
2022-05-02 18:09 ` Borislav Petkov
2022-04-21 14:10 ` [PATCH V6 4/8] x86/entry: Move cld to the start of idtentry macro Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 5/8] x86/entry: Don't call error_entry() for XENPV Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 6/8] x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS Lai Jiangshan
2022-04-29 9:56 ` Borislav Petkov
2022-04-29 11:45 ` Lai Jiangshan
2022-04-29 12:22 ` Borislav Petkov
2022-05-02 12:18 ` Juergen Gross
2022-05-02 17:56 ` Borislav Petkov
2022-05-03 10:43 ` Borislav Petkov
2022-04-21 14:10 ` [PATCH V6 7/8] x86/entry: Remove the branch in sync_regs() Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 8/8] x86/entry: Use idtentry macro for entry_INT80_compat() Lai Jiangshan
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