On Wed, Apr 10, 2024 at 3:50 PM Jason Andryuk <jason.andr...@amd.com> wrote: > > The PVH entrypoint is 32bit non-PIC code running the uncompressed > vmlinux at its load address CONFIG_PHYSICAL_START - default 0x1000000 > (16MB). The kernel is loaded at that physical address inside the VM by > the VMM software (Xen/QEMU). > > When running a Xen PVH Dom0, the host reserved addresses are mapped 1-1 > into the PVH container. There exist system firmwares (Coreboot/EDK2) > with reserved memory at 16MB. This creates a conflict where the PVH > kernel cannot be loaded at that address. > > Modify the PVH entrypoint to be position-indepedent to allow flexibility > in load address. Only the 64bit entry path is converted. A 32bit > kernel is not PIC, so calling into other parts of the kernel, like > xen_prepare_pvh() and mk_pgtable_32(), don't work properly when > relocated. > > This makes the code PIC, but the page tables need to be updated as well > to handle running from the kernel high map. > > The UNWIND_HINT_END_OF_STACK is to silence: > vmlinux.o: warning: objtool: pvh_start_xen+0x7f: unreachable instruction > after the lret into 64bit code. > > Signed-off-by: Jason Andryuk <jason.andr...@amd.com> > --- > --- > arch/x86/platform/pvh/head.S | 44 ++++++++++++++++++++++++++++-------- > 1 file changed, 34 insertions(+), 10 deletions(-) > > diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S > index f7235ef87bc3..bb1e582e32b1 100644 > --- a/arch/x86/platform/pvh/head.S > +++ b/arch/x86/platform/pvh/head.S > @@ -7,6 +7,7 @@ > .code32 > .text > #define _pa(x) ((x) - __START_KERNEL_map) > +#define rva(x) ((x) - pvh_start_xen) > > #include <linux/elfnote.h> > #include <linux/init.h> > @@ -54,7 +55,25 @@ SYM_CODE_START_LOCAL(pvh_start_xen) > UNWIND_HINT_END_OF_STACK > cld > > - lgdt (_pa(gdt)) > + /* > + * See the comment for startup_32 for more details. We need to > + * execute a call to get the execution address to be position > + * independent, but we don't have a stack. Save and restore the > + * magic field of start_info in ebx, and use that as the stack. > + */ > + mov (%ebx), %eax > + leal 4(%ebx), %esp > + ANNOTATE_INTRA_FUNCTION_CALL > + call 1f > +1: popl %ebp > + mov %eax, (%ebx) > + subl $rva(1b), %ebp > + movl $0, %esp > + > + leal rva(gdt)(%ebp), %eax > + leal rva(gdt_start)(%ebp), %ecx > + movl %ecx, 2(%eax) > + lgdt (%eax) > > mov $PVH_DS_SEL,%eax > mov %eax,%ds > @@ -62,14 +81,14 @@ SYM_CODE_START_LOCAL(pvh_start_xen) > mov %eax,%ss > > /* Stash hvm_start_info. */ > - mov $_pa(pvh_start_info), %edi > + leal rva(pvh_start_info)(%ebp), %edi > mov %ebx, %esi > - mov _pa(pvh_start_info_sz), %ecx > + movl rva(pvh_start_info_sz)(%ebp), %ecx > shr $2,%ecx > rep > movsl > > - mov $_pa(early_stack_end), %esp > + leal rva(early_stack_end)(%ebp), %esp > > /* Enable PAE mode. */ > mov %cr4, %eax > @@ -84,28 +103,33 @@ SYM_CODE_START_LOCAL(pvh_start_xen) > wrmsr > > /* Enable pre-constructed page tables. */ > - mov $_pa(init_top_pgt), %eax > + leal rva(init_top_pgt)(%ebp), %eax > mov %eax, %cr3 > mov $(X86_CR0_PG | X86_CR0_PE), %eax > mov %eax, %cr0 > > /* Jump to 64-bit mode. */ > - ljmp $PVH_CS_SEL, $_pa(1f) > + pushl $PVH_CS_SEL > + leal rva(1f)(%ebp), %eax > + pushl %eax > + lretl > > /* 64-bit entry point. */ > .code64 > 1: > + UNWIND_HINT_END_OF_STACK > + > /* Set base address in stack canary descriptor. */ > mov $MSR_GS_BASE,%ecx > - mov $_pa(canary), %eax > + leal rva(canary)(%ebp), %eax
Since this is in 64-bit mode, RIP-relative addressing can be used. > xor %edx, %edx > wrmsr > > call xen_prepare_pvh > > /* startup_64 expects boot_params in %rsi. */ > - mov $_pa(pvh_bootparams), %rsi > - mov $_pa(startup_64), %rax > + lea rva(pvh_bootparams)(%ebp), %rsi > + lea rva(startup_64)(%ebp), %rax RIP-relative here too. > ANNOTATE_RETPOLINE_SAFE > jmp *%rax > > @@ -143,7 +167,7 @@ SYM_CODE_END(pvh_start_xen) > .balign 8 > SYM_DATA_START_LOCAL(gdt) > .word gdt_end - gdt_start > - .long _pa(gdt_start) > + .long _pa(gdt_start) /* x86-64 will overwrite if relocated. */ > .word 0 > SYM_DATA_END(gdt) > SYM_DATA_START_LOCAL(gdt_start) > -- > 2.44.0 > > Brian Gerst
