On 13 May 2018 at 12:58, Michael Zimmermann <[email protected]> wrote: >> No, the other way around. You should raise the TPL to TPL_HIGH_LEVEL >> to prevent being interrupted by something that may corrupt the NEON >> registers. > But isn't that only necessary if you assume that interrupt-handlers use VFP > registers?
Event handlers are called from the timer interrupt handler. So unless you want to restrict use of the NEON to non-event handler context (which is not generally possible for libraries), you will need to raise the TPL to avoid any interruptions. > afaik on ARM <TPL_HIGH_LEVEL events are never called from the timer > interrupt handler so basically if you're going to be interrupted during VFP > operations no other <TPL_HIGH_LEVEL code should ever run. > > Please correct me if I'm misunderstanding something. I don't follow. Your NEON code running at TPL_APPLICATION may be interrupted at any time by event handlers running at higher TPL levels. If such code uses the NEON, it will corrupt your register file. > On Sun, May 13, 2018 at 12:16 PM Ard Biesheuvel <[email protected]> > wrote: > >> On 13 May 2018 at 11:48, Michael Zimmermann <[email protected]> > wrote: >> > So basically using them should be safe as long as you're in >> > EfiGetCurrentTpl() < TPL_HIGH_LEVEL, right? > >> No, the other way around. You should raise the TPL to TPL_HIGH_LEVEL >> to prevent being interrupted by something that may corrupt the NEON >> registers. > >> > Also, it'd probably be trivial to add VFP/NEON regs to >> > EFI_SYSTEM_CONTEXT_ARM though that wouldn't help when writing apps for >> > existing uefi platforms. > >> EFI_SYSTEM_CONTEXT_ARM is covered by the UEFI spec, so that is not >> going to change. > >> > On Sun, May 13, 2018 at 9:32 AM Ard Biesheuvel < > [email protected]> >> > wrote: >> > >> >> On 12 May 2018 at 23:11, Michael Zimmermann <[email protected]> >> > wrote: >> >> > For AArch32 the spec says in 2.3.5.3: >> >> >> Floating point, SIMD, vector operations and other instruction set >> >> > extensions must not >> >> > be used. >> >> > >> >> > For AArch64 the spec says in 2.3.6.4: >> >> >> Floating point and SIMD instructions may be used. >> >> > >> >> > So is there a reason why AArch32 is not allowed to use Floating point >> >> > operations? >> >> > I'd understand if this restriction was limited to runtime services > only >> > but >> >> > I don't see how it makes sense for boot services. >> >> > >> >> > I've written a patch which adds NEON support to FrameBufferBltLib to >> >> > increase the rendering performance(by a lot actually) for 24bit > displays >> >> > and thought about sending it to the mailing list - that's why the >> > question >> >> > came up. >> >> > >> > >> >> The reason for the difference between AArch64 and the other EFI >> >> architectures is that AArch64 does not have a softfloat ABI, so it is >> >> impossible to compile floating point code [portably] without enabling >> >> VFP/NEON. This is why AArch64 is the exception here. >> > >> >> Currently, the AArch32 CPU context structure [EFI_SYSTEM_CONTEXT_ARM] >> >> does not cover VFP/NEON registers, and so they are not >> >> preserved/restored when an interrupt is taken. This means you cannot >> >> use VFP/NEON registers in an event handler or you will corrupt the >> >> VFP/NEON state of the interrupted context. _______________________________________________ edk2-devel mailing list [email protected] https://lists.01.org/mailman/listinfo/edk2-devel
