Re: WARNING: can't access registers at asm_common_interrupt

[Jürgen Groß]

On 13.11.20 18:34, Andy Lutomirski wrote:

On Wed, Nov 11, 2020 at 12:25 PM Andrew Cooper
<andrew.coop...@citrix.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)

I am wondering whether we really want a new feature (basically "not
XENPV). We could use ~X86_FEATURE_XENPV and teach apply_alternatives()
to understand negated features (yes, this limits the number of features
to 32767, but I don't think this is a real problem for quite some time).

Thoughts?


Juergen

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