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

From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
	Brian Gerst <brgerst@gmail.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Borislav Petkov <bp@alien8.de>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address
Date: Tue, 31 Mar 2015 18:43:37 +0200	[thread overview]
Message-ID: <20150331164337.GA8462@gmail.com> (raw)
In-Reply-To: <CAK1hOcOOQAq1sWdnK+nTZBightFaJWiTeQCmCPzhG0-y-UYvOw@mail.gmail.com>

* Denys Vlasenko <vda.linux@googlemail.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