On Fri, 27 Dec 2019 at 05:34, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > On Thu, Dec 26, 2019 at 04:14:07PM +0100, Ard Biesheuvel wrote: > > Calling 32-bit EFI runtime services from a 64-bit OS involves > > switching back to the flat mapping with a stack carved out of > > memory that is 32-bit addressable. > > > > There is no need to actually execute the 64-bit part of this > > routine from the flat mapping as well, as long as the entry > > and return address fit in 32 bits. There is also no need to > > preserve part of the calling context in global variables: we > > can simply preserve the old stack pointer in %r11 across the > > call into 32-bit firmware, and use either stack to preserve > > other values. > > > > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > > --- > > arch/x86/platform/efi/efi_thunk_64.S | 106 ++++++-------------- > > 1 file changed, 29 insertions(+), 77 deletions(-) > > > > diff --git a/arch/x86/platform/efi/efi_thunk_64.S b/arch/x86/platform/efi/efi_thunk_64.S > > index 3189f1394701..7357808d3ae8 100644 > > --- a/arch/x86/platform/efi/efi_thunk_64.S > > +++ b/arch/x86/platform/efi/efi_thunk_64.S > > + /* > > + * Convert 32-bit status code into 64-bit. > > + */ > > +1: btrl $31, %eax > > + jb 3f > > +2: mov %r11, %rsp > > pop %rbx > > movl %ebx, %ss > > pop %rbx > > movl %ebx, %es > > pop %rbx > > movl %ebx, %ds > > - > > - /* > > - * Convert 32-bit status code into 64-bit. > > - */ > > - test %rax, %rax > > - jz 1f > > - movl %eax, %ecx > > - andl $0x0fffffff, %ecx > > - andl $0xf0000000, %eax > > - shl $32, %rax > > - or %rcx, %rax > > -1: > > Is it worth optimizing the conversion? The entire high nibble is > significant according to the spec. It probably doesn't matter except in > one potential case: according to the spec, transitioning secure boot > status to setup mode by deleting the platform key is allowed to return > EFI_WARN_RESET_REQUIRED and AFAICT this can take place after > ExitBootServices? > In theory, yes. But all this code does is move the top bit from bit 31 to 63, which should be fine to convert any return code you may receive from a runtime service. > Separately, it might be worth considering moving the status translation > into C instead of asm for the version in > arch/x86/boot/compressed/efi_thunk_64.S -- there are (at least) three > protocols that have methods that don't return efi_status_t: > DEVICE_PATH_UTILITIES, DEVICE_PATH_TO_TEXT and DEVICE_PATH_FROM_TEXT. If > we ever want to use them (eg for debugging), it might be worth having a > thunk that doesn't mangle the return value. Good point. I'll put that on my todo list. Thanks, Ard.
