Re: [PATCH 1/2] Create UV efi_call macros

Alex Thorlton
2016-05-17T20:14:01Z

On Tue, May 17, 2016 at 01:11:22PM +0100, Matt Fleming wrote:
> On Mon, 16 May, at 05:58:40PM, Alex Thorlton wrote:
> >
> > I was simply re-using the efi_call implementation. Boris suggested that
> > I re-write this using the efi_call_virt macro, so I just went with that.
> > It all seems to work just fine, so I don't see much reason to stray away
> > from that implementation. That being said, I'm obviously not a huge fun
> > of the code duplication across the macros. I think there's probably a
> > way to minimize this, though I haven't quite worked out the best method
> > yet (ideas are welcome :)
>
> The reason I'm pressing for details is that we have a related issue
> with the EFI thunking code (CONFIG_EFI_MIXED), where the function
> pointer we want to call isn't accessible via the EFI System Table, see
> efi_thunk().
>
> Well, technically it *is* accessible, you just can't dereference the
> services at runtime because the pointers in the tables are not 64-bit.
>
> But the same constraints exist for EFI thunk and UV code; given a
> function pointer to execute that isn't in efi.systab, setup the EFI
> runtime environment and call a custom ABI function.

I took a look at this, and see what you mean. You pass in the same
pointer to efi_thunk, which handles essentially the same setup
stuff as efi_call_virt (sync low mappings, disable interrupts, switch
page tables), sans a few of the finer details in
arch_efi_call_virt_setup.

The separate efi_thunk macro is necessary in this case, because you
need to use the efi64_thunk call, with your runtime_service32 massaged
pointer, instead of efi_call, with a pointer straight out of
systab->runtime. This is a similar scenario to ours, in that we
need uv_efi_call instead of efi_call, with our own pointer, instead of
systab->runtime.

The only difference here is that your efi64_thunk call needs a
slightly different setup/teardown than the regular efi_call, so you
need that efi_thunk to be hacked up a bit more (compared to
efi_call_virt) than my uv_efi_call_virt macro.

IINM, we could probably make up for this discrepancy by having a
different arch_efi_call_virt_setup/teardown for the !efi_is_native case
(not sure if that is a feasible idea, correct me if that's stupid).

> I haven't tested this (or thought through all the implications), but
> could you look at providing a table (or something) for mapping a
> function name to a ptr,func pair, e.g.
>
> thunk_get_time: runtime_services32(get_time), efi64_thunk
> thunk_set_time: runtime_services32(set_time), efi64_thunk
> ...
> uv_call_func: efi.uv_systab->function, uv_efi_call_virt
>
> which we could use in arch_efi_call_virt()? That should give us much
> less code duplication and hide all this inside arch/x86.

This sounds like it could be a good way to handle this. I will need to
think about it. Unless someone can say for certain that we can use the
same arch_efi_call_virt_setup/teardown for your efi64_thunk call that we
use for efi_call, then we'll also have to take that into account, which
might make things uglier. Not horrible, but more complicated.

I'm starting to play with this over here to see if I can get a cleaner
implementation working.

Let me know if you have other thoughts. Thanks for the input!

- Alex