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

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Message-Id: <53B49BD3-6F9C-4A78-B203-1BD53034014D@amacapital.net>
Date:   Fri, 8 Nov 2019 19:32:55 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 Nov 8, 2019, at 3:45 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> 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

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.

> 
> 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.

I agree with this.

> 
> 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