linux-kernel - Re: [RFC PATCH] x86/cpu: Fix MSR value truncation issue

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Date:	Wed, 11 Nov 2015 17:05:15 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH] x86/cpu: Fix MSR value truncation issue

On Wed, Nov 11, 2015 at 07:50:04AM -0800, Andy Lutomirski wrote:

> Not terribly surprising :)  Someone (I forget who) told me that 32-bit
> SYSCALL (native 32-bit, not compat) was so full of errata that it was
> unusable.  Even without errata, I don't really see how it would work
> well

No, showstopper appears much earlier: it is only supported on AMD. Which
would mean, yet another vendor special-handling. And I don't think it's
worth it.

Yeah, yeah, it might still be faster than SYSENTER, but 32-bit?! Srsly?!
I'm surprised that thing still builds even. :-)

> -- there's no MSR_SYSCALL_MASK,

Of course there is:

MSRC000_0084 SYSCALL Flag Mask (SYSCALL_FLAG_MASK):

31:0 - Mask: SYSCALL flag mask. Read-write. Reset: 0000_0000h. This register holds the EFLAGS
mask used by the SYSCALL instruction. 1=Clear the corresponding EFLAGS bit when executing the
SYSCALL instruction.

Intel has that too, except again, no SYSCALL in legacy mode on Intel.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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