On Thu, 13 Feb 2020 at 18:23, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > On Thu, Feb 13, 2020 at 03:59:28PM +0100, Ard Biesheuvel wrote: > > Add support for booting 64-bit x86 kernels from 32-bit firmware running > > on 64-bit capable CPUs without requiring the bootloader to implement > > the EFI handover protocol, allocate the setup block etc etc, all of > > which can be done by the stub using code we have already implemented. > > > > Instead, create an ordinary EFI application entrypoint but implemented > > in 32-bit code, so that it can be invoked by 32-bit firmware, and stash > > the address of this 32-bit entrypoint in the .compat section where the > > bootloader can find it. > > > > Note that we use the setup block embedded in the binary to go through > > startup_32(), but it gets reallocated and copied in efi_pe_entry(), > > using the same code that runs when the x86 kernel is booted in EFI > > mode from native firmware. This requires the loaded image protocol to > > be installed on the kernel image's EFI handle, and point to the kernel > > image itself and not to its loader. This, in turn, requires the > > bootloader to use the LoadImage() boot services to load the 64-bit > > image from 32-bit firmware, which is in fact supported by firmware > > based on EDK2. (Only StartImage() will fail, and instead, the newly > > added entrypoint needs to be invoked) > > > > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > > --- > > arch/x86/boot/compressed/head_64.S | 61 +++++++++++++++++++- > > 1 file changed, 59 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S > > index a4f5561c1c0e..7baaf9c97f5a 100644 > > --- a/arch/x86/boot/compressed/head_64.S > > +++ b/arch/x86/boot/compressed/head_64.S > > @@ -207,8 +207,8 @@ SYM_FUNC_START(startup_32) > > cmp $0, %edi > > jz 1f > > leal efi64_stub_entry(%ebp), %eax > > - movl %esi, %edx > > movl efi32_boot_args+4(%ebp), %esi > > + movl efi32_boot_args+8(%ebp), %edx // saved bootparams pointer > > 1: > > #endif > > pushl %eax > > @@ -233,6 +233,8 @@ SYM_FUNC_START(efi32_stub_entry) > > 1: pop %ebp > > subl $1b, %ebp > > > > + movl %esi, efi32_boot_args+8(%ebp) > > +2: > > I think it would be easier to read if this were turned into a proper > SYM_INNER_LABEL. You could then also just put efi32_pe_entry into the > .text section instead of moving it to the end of .head.text with > .subsection. Another option is to use a separate flag in .data to > indicate that we entered via efi32_pe_entry, then you could just jump > to efi32_stub_entry. > Indeed. > > movl %ecx, efi32_boot_args(%ebp) > > movl %edx, efi32_boot_args+4(%ebp) > > movb $0, efi_is64(%ebp) > > @@ -249,6 +251,49 @@ SYM_FUNC_START(efi32_stub_entry) > > > > jmp startup_32 > > SYM_FUNC_END(efi32_stub_entry) > > + > > +#define ST32_boottime 60 // offsetof(efi_system_table_32_t, boottime) > > +#define BS32_handle_protocol 88 // offsetof(efi_boot_services_32_t, handle_protocol) > > +#define LI32_image_base 32 // offsetof(efi_loaded_image_32_t, image_base) > > + > > + .subsection 1 > > + .code32 > > +SYM_FUNC_START(efi32_pe_entry) > > + pushl %ebp > > + > > + call 3f > > +3: pop %ebp > > + subl $3b, %ebp > > + > > + /* Get the loaded image protocol pointer from the image handle */ > > + subl $12, %esp // space for the loaded image pointer > > + pushl %esp // pass its address > > + leal 5f(%ebp), %eax > > + pushl %eax // pass the GUID address > > + pushl 28(%esp) // pass the image handle > > + > > + movl 36(%esp), %eax // sys_table > > + movl ST32_boottime(%eax), %eax // sys_table->boottime > > + call *BS32_handle_protocol(%eax) // sys_table->boottime->handle_protocol > > + cmp $0, %eax > > + jnz 4f > > + > > + movl 32(%esp), %ecx // image_handle > > + movl 36(%esp), %edx // sys_table > > + movl 12(%esp), %esi // loaded_image > > + movl LI32_image_base(%esi), %esi // loaded_image->image_base > > + jmp 2b > > + > > +4: addl $24, %esp > > + popl %ebp > > + ret > > +SYM_FUNC_END(efi32_pe_entry) > > + > > + /* EFI loaded image protocol GUID */ > > +5: .long 0x5B1B31A1 > > + .word 0x9562, 0x11d2 > > + .byte 0x8E, 0x3F, 0x00, 0xA0, 0xC9, 0x69, 0x72, 0x3B > > + .previous > > Any reason for this not to live in .data (or .rodata)? None other than laziness ... > > #endif > > > > .code64 > > @@ -465,11 +510,23 @@ SYM_CODE_END(startup_64) > > SYM_FUNC_START(efi64_stub_entry) > > SYM_FUNC_START_ALIAS(efi_stub_entry) > > and $~0xf, %rsp /* realign the stack */ > > +#ifdef CONFIG_EFI_MIXED > > + cmpl $0, %edx > > + jz 0f > > +#endif > > call efi_main > > movq %rax,%rsi > > movl BP_code32_start(%esi), %eax > > leaq startup_64(%rax), %rax > > jmp *%rax > > + > > +#ifdef CONFIG_EFI_MIXED > > +0: movl %edi, %ecx // MS calling convention > > + movl %esi, %edx > > + call efi_pe_entry > > +1: hlt > > + jmp 1b > > +#endif > > SYM_FUNC_END(efi64_stub_entry) > > SYM_FUNC_END_ALIAS(efi_stub_entry) > > #endif > > @@ -641,7 +698,7 @@ SYM_DATA_START_LOCAL(gdt) > > SYM_DATA_END_LABEL(gdt, SYM_L_LOCAL, gdt_end) > > > > #ifdef CONFIG_EFI_MIXED > > -SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0) > > +SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0, 0) > > SYM_DATA(efi_is64, .byte 1) > > #endif > > > > -- > > 2.17.1 > >
