Re: [PATCH V6 3/8] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()

[Juergen Gross]

On 28.04.22 02:33, Lai Jiangshan wrote:
> On Thu, Apr 28, 2022 at 1:45 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
> > On Thu, Apr 21, 2022 at 10:10:50PM +0800, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
> > >
> > > 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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