-----Original Message-----
From: Segher Boessenkool <redacted>
Sent: 10 September 2020 16:21
To: David Laight <redacted>
Cc: 'Christophe Leroy' <redacted>; 'Linus Torvalds' <torvalds@linux-
foundation.org>; linux-arch [off-list ref]; Kees Cook [off-list ref]; the
arch/x86 maintainers [off-list ref]; Nick Desaulniers [off-list ref]; Linux Kernel
Mailing List [off-list ref]; Alexey Dobriyan [off-list ref]; Luis Chamberlain
[off-list ref]; Al Viro [off-list ref]; linux-fsdevel [off-list ref];
linuxppc-dev [off-list ref]; Christoph Hellwig [off-list ref]
Subject: Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3

On Thu, Sep 10, 2020 at 12:26:53PM +0000, David Laight wrote:

quoted

Actually this is pretty sound:
	__label__ label;
	register int eax asm ("eax");
	// Ensure eax can't be reloaded from anywhere
	// In particular it can't be reloaded after the asm goto line
	asm volatile ("" : "=r" (eax));

This asm is fine.  It says it writes the "eax" variable, which lives in
the eax register *in that asm* (so *not* guaranteed after it!).

quoted

	// Provided gcc doesn't save eax here...
	asm volatile goto ("xxxxx" ::: "eax" : label);

So this is incorrect.

From the other email:

It is neither input nor output operand here!  Only *then* is a local
register asm guaranteed to be in the given reg: as input or output to an
inline asm.

Ok, so adding '"r" (eax)' to the input section helps a bit.

quoted

	// ... and reload the saved value here.
	// The input value here will be that modified by the 'asm goto'.
	// Since this modifies eax it can't be moved before the 'asm goto'.
	asm volatile ("" : "+r" (eax));
	// So here eax must contain the value set by the "xxxxx" instructions.

No, the register eax will contain the value of the eax variable.  In the
asm; it might well be there before or after the asm as well, but none of
that is guaranteed.

Perhaps not 'guaranteed', but very unlikely to be wrong.
It doesn't give gcc much scope for not generating the desired code.

	David

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