On Fri, Feb 02, 2018 at 10:06:07AM +0000, Ard Biesheuvel wrote: > On 31 January 2018 at 10:40, Laszlo Ersek <ler...@redhat.com> wrote: > > On 01/30/18 23:25, Kinney, Michael D wrote: > >> Laszlo, > >> > >> I agree that the function is better than a macro. > >> > >> I thought of the alignment issues as well. CopyMem() > >> is a good solution. We could also consider > >> WriteUnalignedxx() functions in BaseLib. > > > > IMO, the WriteUnalignedxx functions are a bit pointless in the exact > > form they are declared (this was discussed earlier esp. with regard to > > aarch64). The functions take pointers to objects that already have the > > target type, such as > > > > UINT32 > > EFIAPI > > WriteUnaligned32 ( > > OUT UINT32 *Buffer, > > IN UINT32 Value > > ) > > > > Here the type of Buffer should be (VOID *), not (UINT32 *). Otherwise, > > the undefined behavior (due to mis-alignment) surfaces as soon as the > > function is called with an unaligned pointer (i.e. before the target > > area is actually written). > > > >> I was originally thinking this functionality would go > >> into BaseLib. But with the use of CopyMem(), we can't > >> do that. > > > > Can we put it in BaseMemoryLib instead (which is where CopyMem() is > > from)? That library class is still low-level enough. And, while I count > > 9 library instances, PatchAssembly() is not a large function, we could > > tolerate adding it to all 9 instances, identically. > > > > Let me also ask the opposite question: should we perhaps make the > > PatchAssembly() API *less* abstract? (Also suggested by your naming of > > the macro, PATCH_X86_ASM.) If the instruction encoding on e.g. AARCH64 > > doesn't lend itself to such patching (= expressed through the address > > right after the instruction), then even BaseMemoryLib may be too generic > > for the API. > > > >> Maybe we should use WriteUnalignedxx() and > >> add some ASSERT() checks. > >> > >> VOID > >> PatchAssembly ( > >> VOID *BufferEnd, > >> UINT64 PatchValue, > >> UINTN ValueSize > >> ) > >> { > >> ASSERT ((UINTN)BufferEnd > ValueSize); > >> switch (ValueSize) { > >> case 1: > >> ASSERT (PatchValue <= MAX_UINT8); > >> *((UINT8 *)BufferEnd - 1) = (UINT8)PatchValue; > >> case 2: > >> ASSERT (PatchValue <= MAX_UINT16); > >> WriteUnaligned16 ((UINT16 *)(BufferEnd) - 1, (UINT16)PatchValue)); > >> break; > >> case 4: > >> ASSERT (PatchValue <= MAX_UINT32); > >> WriteUnaligned32 ((UINT32 *)(BufferEnd) - 1, (UINT32)PatchValue)); > >> break; > >> case 8: > >> WriteUnaligned64 ((UINT64 *)(BufferEnd) - 1, PatchValue)); > >> break; > >> default: > >> ASSERT (FALSE); > >> } > >> } > > > > In my opinion: > > > > - If Ard and Leif say that PatchAssembly() API makes sense for AARCH64, > > then I think we can go with the above generic implementation (for > > BaseLib). > > > > Code patching on ARM/AARCH64 has some hoops to jump through, i.e., > clean the D-cache to the point of unification, invalidate the I-cache, > probably some barriers in case the patching function happened to end > up in the same cache line as the patchee
Not just the same cache line. Prefetching can happen whenever, for whatever reason. > (which may not be a concern > for this specific use case, but it does need to be taken into account > if this is turned into a patch-any-assembly-anywhere function) > > So if the PatchAssembly() prototype does end up in a generic library > class, we'd have to provide ARM and AARCH64 specific implementations > anyway, and given that I don't see any use for this on ARM/AARCH64 in > the first place, I think this should belong in an IA32/X64 specific > package. I also don't see a specific use for this on ARM* at the moment. But if this is going to become more widespread, it would be useful to introduce a higher-level layer with more portable semantics (I don't know RISC-V, but could imagine they require similar). However, at that point, we would probably want something buffer-oriented rather than instruction-oriented, since we'd like to keep the overhead down if writing more than one register's worth. / Leif _______________________________________________ edk2-devel mailing list edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel