linux-kernel - Re: [PATCH] x86/asm/entry/64: better check for canonical address

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Date:	Tue, 31 Mar 2015 18:43:37 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address


* Denys Vlasenko <vda.linux@...glemail.com> wrote:

> > I guess they could optimize it by adding a single "I am a modern 
> > OS executing regular userspace" flag to the descriptor [or 
> > expressing the same as a separate instruction], to avoid all that 
> > legacy crap that won't trigger on like 99.999999% of systems ...
> 
> Yes, that would be a useful addition. Interrupt servicing on x86 
> takes a non-negligible hit because of IRET slowness.

But ... to react to your other patch: detecting the common easy case 
and doing a POPF+RET ourselves ought to be pretty good as well?

But only if ptregs->rip != the magic RET itself, to avoid recursion.

Even with all those extra checks it should still be much faster.

Thanks,

	Ingo
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