From: Peter Zijlstra <peterz@infradead.org>
To: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
Borislav Petkov <bp@alien8.de>,
Lai Jiangshan <jiangshan.ljs@antgroup.com>,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH V3 6/7] x86/entry: Don't call error_entry for XENPV
Date: Wed, 16 Mar 2022 16:09:28 +0100 [thread overview]
Message-ID: <YjH9qCkeCXZ4XjLx@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20220315073949.7541-7-jiangshanlai@gmail.com>
On Tue, Mar 15, 2022 at 03:39:48PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@antgroup.com>
>
> When in XENPV, it is already in the task stack, and it can't fault
> for native_iret() nor native_load_gs_index() since XENPV uses its own
> pvops for iret and load_gs_index(). And it doesn't need to switch CR3.
> So there is no reason to call error_entry() in XENPV.
>
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> ---
> arch/x86/entry/entry_64.S | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index e4a07276fd1c..ec885c2107de 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -328,8 +328,17 @@ SYM_CODE_END(ret_from_fork)
> PUSH_AND_CLEAR_REGS
> ENCODE_FRAME_POINTER
>
> - call error_entry
> - movq %rax, %rsp /* switch stack settled by sync_regs() */
> + /*
> + * Call error_entry and switch stack settled by sync_regs().
> + *
> + * When in XENPV, it is already in the task stack, and it can't fault
> + * for native_iret() nor native_load_gs_index() since XENPV uses its
> + * own pvops for iret and load_gs_index(). And it doesn't need to
> + * switch CR3. So it can skip invoking error_entry().
> + */
> + ALTERNATIVE "call error_entry; movq %rax, %rsp", \
> + "", X86_FEATURE_XENPV
> +
> ENCODE_FRAME_POINTER
> UNWIND_HINT_REGS
>
Oooh, here we go, this is the answer to my question for patch #1, a note
in the changelog might be nice. Something like:
"This looses a Xen PV optimization, which will be restored in a later
patch. The superfluous stack switch is just that."
next prev parent reply other threads:[~2022-03-16 15:11 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-15 7:39 [PATCH V3 0/7] x86/entry: Clean up entry code Lai Jiangshan
2022-03-15 7:39 ` [PATCH V3 1/7] x86/entry: Use idtentry macro for entry_INT80_compat Lai Jiangshan
2022-03-16 13:48 ` Peter Zijlstra
2022-03-15 7:39 ` [PATCH V3 2/7] x86/traps: Move pt_regs only in fixup_bad_iret() Lai Jiangshan
2022-03-15 7:39 ` [PATCH V3 3/7] x86/entry: Switch the stack after error_entry() returns Lai Jiangshan
2022-03-16 2:37 ` Josh Poimboeuf
2022-03-16 3:02 ` Lai Jiangshan
2022-03-16 15:05 ` Peter Zijlstra
2022-03-16 16:43 ` Lai Jiangshan
2022-03-18 11:12 ` Peter Zijlstra
2022-03-15 7:39 ` [PATCH V3 4/7] x86/entry: move PUSH_AND_CLEAR_REGS out of error_entry Lai Jiangshan
2022-03-16 15:07 ` Peter Zijlstra
2022-03-16 16:18 ` Lai Jiangshan
2022-03-15 7:39 ` [PATCH V3 5/7] x86/entry: Move cld to the start of idtentry Lai Jiangshan
2022-03-15 7:39 ` [PATCH V3 6/7] x86/entry: Don't call error_entry for XENPV Lai Jiangshan
2022-03-16 2:59 ` Josh Poimboeuf
2022-03-16 15:09 ` Peter Zijlstra [this message]
2022-03-16 16:48 ` Lai Jiangshan
2022-03-15 7:39 ` [PATCH V3 7/7] x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS Lai Jiangshan
2022-03-16 15:12 ` [PATCH V3 0/7] x86/entry: Clean up entry code Peter Zijlstra
2022-03-16 15:13 ` Peter Zijlstra
2022-03-17 12:58 ` Lai Jiangshan
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