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

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Date:   Sat, 9 Nov 2019 00:45:47 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch 4/9] x86/io: Speedup schedule out of I/O bitmap user

On Fri, 8 Nov 2019, Andy Lutomirski wrote:
> On 11/7/19 6:08 AM, Thomas Gleixner wrote:
> > On Thu, 7 Nov 2019, Thomas Gleixner wrote:
> >> Just that I can't add the storage to tss_struct due to the VMX insanity of
> >> setting TSS limit hard to 0x67 on vmexit instead of restoring the host
> >> value.
> > 
> > Well, I can. The build bugon in vmx.c is just bogus.
> 
> 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

2) The IO_BITMAP_OFFSET is not the right thing to check because it makes
   asssumptions about the layout of tss_struct. Nothing requires that the
   I/O bitmap is placed right after x86_tss, which is the hardware mandated
   tss structure. It pointlessly makes restrictions on the struct
   tss_struct layout.

The proper thing to check is:

    - Offset of x86_tss in tss_struct is 0
    - Size of x86_tss == 0x68

We already have the page alignment sanity check off TSS in
cpu_entry_area.c. That's where this should have gone into in the first
place.

Thanks,

	tglx