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

Thomas Gleixner
2019-11-08T23:45:56Z

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