Date:   Fri, 13 Nov 2020 09:34:57 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Shinichiro Kawasaki <shinichiro.kawasaki@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Juergen Gross <jgross@...e.com>, X86 ML <x86@...nel.org>
Subject: Re: WARNING: can't access registers at asm_common_interrupt

On Wed, Nov 11, 2020 at 12:25 PM Andrew Cooper
<andrew.cooper3@...rix.com> wrote:
>
> On 11/11/2020 20:15, Josh Poimboeuf wrote:
> > On Wed, Nov 11, 2020 at 09:07:30PM +0100, Peter Zijlstra wrote:
> >> On Wed, Nov 11, 2020 at 01:59:00PM -0600, Josh Poimboeuf wrote:
> >>> On Wed, Nov 11, 2020 at 08:42:06PM +0100, Peter Zijlstra wrote:
> >>>>> Would objtool have an easier time coping if this were implemented in
> >>>>> terms of a static call?
> >>>> I doubt it, the big problem is that there is no visibility into the
> >>>> actual alternative text. Runtime patching fragments into static call
> >>>> would have the exact same problem.
> >>>>
> >>>> Something that _might_ maybe work is trying to morph the immediate
> >>>> fragments into an alternative. That is, instead of this:
> >>>>
> >>>> static inline notrace unsigned long arch_local_save_flags(void)
> >>>> {
> >>>>    return PVOP_CALLEE0(unsigned long, irq.save_fl);
> >>>> }
> >>>>
> >>>> Write it something like:
> >>>>
> >>>> static inline notrace unsigned long arch_local_save_flags(void)
> >>>> {
> >>>>    PVOP_CALL_ARGS;
> >>>>    PVOP_TEST_NULL(irq.save_fl);
> >>>>    asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CALL),
> >>>>                                    "PUSHF; POP _ASM_AX",
> >>>>                                    X86_FEATURE_NATIVE)
> >>>>                        : CLBR_RET_REG, ASM_CALL_CONSTRAINT
> >>>>                        : paravirt_type(irq.save_fl.func),
> >>>>                          paravirt_clobber(PVOP_CALLEE_CLOBBERS)
> >>>>                        : "memory", "cc");
> >>>>    return __eax;
> >>>> }
> >>>>
> >>>> And then we have to teach objtool how to deal with conflicting
> >>>> alternatives...
> >>>>
> >>>> That would remove most (all, if we can figure out a form that deals with
> >>>> the spinlock fragments) of paravirt_patch.c
> >>>>
> >>>> Hmm?
> >>> I was going to suggest something similar.  Though I would try to take it
> >>> further and replace paravirt_patch_default() with static calls.
> >> Possible, we just need to be _really_ careful to not allow changing
> >> those static_call()s. So maybe we need DEFINE_STATIC_CALL_RO() which
> >> does a __ro_after_init on the whole thing.
> > But what if you want to live migrate to another hypervisor ;-)
>
> The same as what happens currently.  The user gets to keep all the
> resulting pieces ;)
>
> >>> Either way it doesn't make objtool's job much easier.  But it would be
> >>> nice to consolidate runtime patching mechanisms and get rid of
> >>> .parainstructions.
> >> I think the above (combining alternative and paravirt/static_call) does
> >> make objtool's job easier, since then we at least have the actual
> >> alternative instructions available to inspect, or am I mis-understanding
> >> things?
> > Right, it makes objtool's job a _little_ easier, since it already knows
> > how to read alternatives.  But it still has to learn to deal with the
> > conflicting stack layouts.
>
> I suppose the needed abstraction is "these blocks will start and end
> with the same stack layout", while allowing the internals to diverge.
>

How much of this stuff is actually useful anymore?  I'm wondering if
we can move most or all of this crud to
cpu_feature_enabled(X86_FEATURE_XENPV) and its asm equivalent.  The
full list, annotated, appears to be:

        const unsigned char     irq_irq_disable[1];

This is CLI or CALL, right?

        const unsigned char     irq_irq_enable[1];

STI or CALL.

        const unsigned char     irq_save_fl[2];

PUSHF; POP %r/eax.  I *think* I read the paravirt mess correctly and
this also turns into CALL.

        const unsigned char     mmu_read_cr2[3];
        const unsigned char     mmu_read_cr3[3];
        const unsigned char     mmu_write_cr3[3];

The write CR3 is so slow that I can't imagine us caring.  Reading CR3
should already be fairly optimized because it's slow on old non-PV
hypervisors, too.  Reading CR2 is rare and lives in asm.  These also
appear to just switch between MOV and CALL, anyway.

        const unsigned char     irq_restore_fl[2];

Ugh, this one sucks.  IMO it should be, for native and PV:

if (flags & X86_EFLAGS_IF) {
  local_irq_enable();  /* or raw? */
} else {
  if (some debugging option) {
    WARN_ON_ONCE(save_fl() & X86_EFLAGS_IF);
  }
}

POPF is slooooow.

        const unsigned char     cpu_wbinvd[2];

This is hilariously slow no matter what.  static_call() or even just a
plain old indirect call should be fine.

        const unsigned char     cpu_usergs_sysret64[6];

This is in the asm and we shouldn't be doing it at all for Xen PV.
IOW we should just drop this patch site entirely.  I can possibly find
some time to get rid of it, and maybe someone from Xen land can help.
I bet that we can gain a lot of perf on Xen PV by cleaning this up,
and I bet it will simplify everything.

        const unsigned char     cpu_swapgs[3];

This is SWAPGS or nop, unless I've missed some subtlety.

        const unsigned char     mov64[3];

This is some PTE magic, and I haven't deciphered it yet.

So I think there is at most one of these that wants anything more
complicated than a plain ALTERNATIVE.  Any volunteers to make it so?
Juergen, if you do all of them except USERGS_SYSRET64, I hereby
volunteer to do that one.

BTW, if y'all want to live migrate between Xen PV and anything else,
you are nuts.

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