[RFC PATCH v4 5/5] arm64: neon: Allow EFI runtime services to use FPSIMD in irq context

[Dave Martin]

On Wed, Aug 02, 2017 at 05:02:36PM +0100, Catalin Marinas wrote:
> On Wed, Aug 02, 2017 at 04:15:21PM +0100, Dave P Martin wrote:
> > On Wed, Aug 02, 2017 at 03:58:01PM +0100, Catalin Marinas wrote:
> > > On Fri, Jul 28, 2017 at 02:50:24PM +0100, Dave P Martin wrote:
> > > > Now that kernel-mode NEON use is non-nestable and not allowed in
> > > > hardirq/nmi context, we have a problem if the kernel decides to
> > > > call into EFI during an interrupt that interrupted a
> > > > kernel_neon_begin()..._end() block.  This will occur if the kernel
> > > > tries to write diagnostic data to EFI persistent storage during
> > > > a panic triggered by an NMI for example.
> > > > 
> > > > EFI runtime services specify an ABI that clobbers the FPSIMD state,
> > > > rather than being able to use it optionally as an accelerator.
> > > > This means that EFI is really a special case and can be handled
> > > > separately.
> > > > 
> > > > To enable EFI calls from interrupts, this patch creates dedicated
> > > > __efi_fpsimd_{begin,end}() helpers solely for this purpose, which
> > > > save/restore to a separate percpu buffer if called in a context
> > > > where kernel_neon_begin() is not usable.
> > > > 
> > > > Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> > > 
> > > Should this patch be reordered with patch 4? It looks like patch 4 no
> > > longer allows EFI runtime services in IRQ context.
> > 
> > Hmmm, yes, patches 4/5 are not bisectable at present.
> > 
> > I can't immediately see why patch 5 won't work if they're swapped:
> > rather, the !may_use_simd() fallback and efi_fpsimd_state won't get
> > used, and save/restore around EFI should work much the same as it does
> > today.
> > 
> > I'm happy to swap the patches, but I don't want to put too much effort
> > into confirming the above.  How concerned are you about the risk?
> 
> I'm not concerned but we could at least test that it boots with patches
> 1-4.

Agreed, I'll be checking that, but I won't otherwise kick it too hard.

Cheers
---Dave