linux-kernel - RE: System Call trashing registers

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Message-ID: <c3b9a46e2cf44ff1a08efadde68248b2@AcuMS.aculab.com>
Date:   Mon, 28 Aug 2023 17:06:10 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Joshua Hudson' <joshudson@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: RE: System Call trashing registers

From: Joshua Hudson
> Sent: 24 August 2023 17:15
> 
> 1) A lot of my old 32-bit programs don't work on x64 linux anymore
> because int 80h now trashes ecx and edx. This hasn't been a serious
> problem for me.

Aren't both ecx and edx caller saved?
So if the code is using asm syscall wrappers provided the asm
wrappers don't expect the registers be saved (which is unlikely)
then it is safe for the kernel to trash them.

OTOH I have seen code compiled with inlined syscall code
(but not recently). In that case it will matter.

It gets more interesting with all the xmm/ymm/zmm registers.
They are also all caller saved, so if the compiler always
sees a real function call wrapping a system call then the kernel
need not save them and can return with them all set to zero.

I don't believe that is done, but it is likely to be a measurable
performance gain for most programs.

	David

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