Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall

[Andy Lutomirski]

On Fri, Oct 1, 2021, at 2:56 AM, Thomas Gleixner wrote:
> On Thu, Sep 30 2021 at 21:41, Andy Lutomirski wrote:
>> On Thu, Sep 30, 2021, at 5:01 PM, Thomas Gleixner wrote:
>
>> Now that I read the docs some more, I'm seriously concerned about this
>> XSAVE design.  XSAVES with UINTR is destructive -- it clears UINV.  If
>> we actually use this, then the whole last_cpu "preserve the state in
>> registers" optimization goes out the window.  So does anything that
>> happens to assume that merely saving the state doesn't destroy it on
>> respectable modern CPUs XRSTORS will #GP if you XRSTORS twice, which
>> makes me nervous and would need a serious audit of our XRSTORS paths.
>
> I have no idea what you are fantasizing about. You can XRSTORS five
> times in a row as long as your XSTATE memory image is correct.

I'm just reading TFM, which is some kind of dystopian fantasy.

11.8.2.4 XRSTORS

Before restoring the user-interrupt state component, XRSTORS verifies that UINV is 0. If it is not, XRSTORS
causes a general-protection fault (#GP) before loading any part of the user-interrupt state component. (UINV
is IA32_UINTR_MISC[39:32]; XRSTORS does not check the contents of the remainder of that MSR.)

So if UINV is set in the memory image and you XRSTORS five times in a row, the first one will work assuming UINV was zero.  The second one will #GP.  And:

11.8.2.3 XSAVES
After saving the user-interrupt state component, XSAVES clears UINV. (UINV is IA32_UINTR_MISC[39:32];
XSAVES does not modify the remainder of that MSR.)

So if we're running a UPID-enabled user task and we switch to a kernel thread, we do XSAVES and UINV is cleared.  Then we switch back to the same task and don't do XRSTORS (or otherwise write IA32_UINTR_MISC) and UINV is still clear.

And we had better clear UINV when running a kernel thread because the UPID might get freed or the kernel thread might do some CPL3 shenanigans (via EFI, perhaps? I don't know if any firmwares actually do this).

So all this seems to put UINV into the "independent" category of feature along with LBR.  And the 512-byte wastes from extra copies of the legacy area and the loss of the XMODIFIED optimization will just be collateral damage.