Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

[Noah Goldstein]

Forgive if this is silly but would it be possible to do something
simliar to rseq where the user can register a set of features for a
program counter region and then on interrupt check that to determine
what needs to be saved?

For example if a user doesn't use any AMX but loads a library that
does, for all ip in the users code AMX state won't be saved but an
interrupt in ip range of the library will save AMX state.

One advantage of this is it would be pretty easy silently do this
right with compiler support and to preserve old code the "ip not found
in table" case could default to the worst case the CPU supports.

On Tue, Mar 30, 2021 at 1:09 AM Andy Lutomirski via Libc-alpha
<libc-alpha@xxxxxxxxxxxxxx> wrote:
>
> On Mon, Mar 29, 2021 at 3:38 PM Len Brown <lenb@xxxxxxxxxx> wrote:
> >
> > On Mon, Mar 29, 2021 at 2:16 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > >
>
> > Hi Andy,
> >
> > Can you provide a concise definition of the exact problemI(s) this thread
> > is attempting to address?
>
> The AVX-512 state, all by itself, is more than 2048 bytes.  Quoting
> the POSIX sigaltstack page (man 3p sigaltstack):
>
>        The  value  SIGSTKSZ is a system default specifying the number of bytes
>        that would be used to cover the usual case when manually allocating  an
>        alternate  stack area. The value MINSIGSTKSZ is defined to be the mini‐
>        mum stack size for a signal handler. In computing  an  alternate  stack
>        size, a program should add that amount to its stack requirements to al‐
>        low for the system implementation overhead. The  constants  SS_ONSTACK,
>        SS_DISABLE, SIGSTKSZ, and MINSIGSTKSZ are defined in <signal.h>.
>
> arch/x86/include/uapi/asm/signal.h:#define MINSIGSTKSZ    2048
> arch/x86/include/uapi/asm/signal.h:#define SIGSTKSZ    8192
>
> Regrettably, the Linux signal frame format is the uncompacted format
> and, also regrettably, the uncompacted format has the nasty property
> that its format depends on XCR0 but not on the set of registers that
> are actually used or wanted, so, with the current ABI, the signal
> frame is stuck being quite large for all programs on a machine that
> supports avx512 and has it enabled by the kernel.  And it's even
> larger for AMX and violates SIGSTKSZ as well as MINSTKSZ.
>
> There are apparently real programs that break as a result.  We need to
> find a way to handle new, large extended states without breaking user
> ABI.  We should also find a way to handle them without consuming silly
> amounts of stack space for programs that don't use them.
>
> Sadly, if the solution we settle on involves context switching XCR0,
> performance on first-generation hardware will suffer because VMX does
> not have any way to allow guests to write XCR0 without exiting.  I
> don't consider this to be a showstopper -- if we end up having this
> problem, fixing it in subsequent CPUs is straightforward.
>
> >
> > Thank ahead-of-time for excluding "blow up power consumption",
> > since that paranoia is not grounded in fact.
> >
>
> I will gladly exclude power consumption from this discussion, since
> that's a separate issue that has nothing to do with the user<->kernel
> ABI.