Re: [patch 4/9] x86/io: Speedup schedule out of I/O bitmap user

Thomas Gleixner
2019-11-10T12:44:05Z

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On Fri, 8 Nov 2019, Andy Lutomirski wrote:
> > On Nov 8, 2019, at 3:45 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > ﻿On Fri, 8 Nov 2019, Andy Lutomirski wrote:
> >> SDM vol 3 27.5.2 says the BUILD_BUG_ON is right. Or am I
> >> misunderstanding you?
> >>
> >> I'm reasonably confident that the TSS limit is indeed 0x67 after VM
> >> exit, and I wrote the existing code that tries to optimize this to avoid
> >> LTR when not needed.
> >
> > The BUILD_BUG_ON(IO_BITMAP_OFFSET - 1 == 0x67) in the VMX code is bogus in
> > two aspects:
> >
> > 1) This wants to be in generic x86 code
>
> I think disagree. The only thing special about 0x67 is that VMX hard
> codes it. It’s specifically a VMX-ism. So I think the VMX code should
> indeed assert that 0x67 is a safe value.

Yes, it is a VMX specific issue, but I really prefer the build to fail in
the common code without having to enable VMX if something changes the
TSS layout in incompatible ways.

Thanks,

tglx