Messages in this thread | | | Date | Wed, 16 Mar 2022 16:09:28 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH V3 6/7] x86/entry: Don't call error_entry for XENPV |
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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."
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