[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen staging] x86/boot: Preserve the value clobbered by the load-base calculation
commit e58a2858d588ed57ca13200f3d3148d78ad0e491 Author: Frediano Ziglio <frediano.ziglio@xxxxxxxxx> AuthorDate: Thu Aug 22 15:00:43 2024 +0100 Commit: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> CommitDate: Tue Aug 27 18:08:19 2024 +0100 x86/boot: Preserve the value clobbered by the load-base calculation Right now, Xen clobbers the value at 0xffc when performing it's load-base calculation. We've got plenty of free registers at this point, so the value can be preserved easily. This fixes a real bug booting under Coreboot+SeaBIOS, where 0xffc happens to be the cbmem pointer (e.g. Coreboot's dmesg ring, among other things). However, there's also a better choice of memory location to use than 0xffc, as all our supported boot protocols have a pointer to an info structure in %ebx. Update the documentation to match. Fixes: 1695e53851e5 ("x86/boot: Fix the boot time relocation calculations") Fixes: d96bb172e8c9 ("x86/entry: Early PVH boot code") Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> --- docs/hypervisor-guide/x86/how-xen-boots.rst | 12 ++++++++--- xen/arch/x86/boot/head.S | 33 +++++++++++++++++++---------- 2 files changed, 31 insertions(+), 14 deletions(-) diff --git a/docs/hypervisor-guide/x86/how-xen-boots.rst b/docs/hypervisor-guide/x86/how-xen-boots.rst index ca77d7c8a3..8b3229005c 100644 --- a/docs/hypervisor-guide/x86/how-xen-boots.rst +++ b/docs/hypervisor-guide/x86/how-xen-boots.rst @@ -96,6 +96,12 @@ Xen, once loaded into memory, identifies its position in order to relocate system structures. For 32bit entrypoints, this necessarily requires a call instruction, and therefore a stack, but none of the ABIs provide one. -Overall, given that on a BIOS-based system, the IVT and BDA occupy the first -5/16ths of the first page of RAM, with the rest free to use, Xen assumes the -top of the page is safe to use. +In each supported 32bit entry protocol, ``%ebx`` is a pointer to an info +structure, and it is highly likely that this structure does not overlap with +Xen. Therefore we use this as a temporary stack, preserving the prior value, +in order to calculate Xen's position in memory. + +If this heuristic happens to be wrong (most likely because we were booted by +some other protocol), the calculation stills works as long as ``%ebx`` points +at RAM and does not alias the currently-executing instructions. This is +reasonably likely, and the best we can manage given no other information. diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S index d8ac0f0494..d867b015d9 100644 --- a/xen/arch/x86/boot/head.S +++ b/xen/arch/x86/boot/head.S @@ -414,17 +414,23 @@ __pvh_start: cli /* - * We need one push/pop to determine load address. Use the same - * absolute stack address as the native path, for lack of a better - * alternative. + * We need one call (i.e. push) to determine the load address. See + * __start for a discussion on how to do this safely using the PVH + * info structure. */ - mov $0x1000, %esp + + /* Preserve the field we're about to clobber. */ + mov (%ebx), %edx + lea 4(%ebx), %esp /* Calculate the load base address. */ call 1f 1: pop %esi sub $sym_offs(1b), %esi + /* Restore the clobbered field. */ + mov %edx, (%ebx) + /* Set up stack. */ lea STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp @@ -460,21 +466,26 @@ __start: /* * Multiboot (both 1 and 2) specify the stack pointer as undefined * when entering in BIOS circumstances. This is unhelpful for - * relocatable images, where one push/pop is required to calculate - * images load address. + * relocatable images, where one call (i.e. push) is required to + * calculate the image's load address. * - * On a BIOS-based system, the IVT and BDA occupy the first 5/16ths of - * the first page of RAM, with the rest free for use. Use the top of - * this page for a temporary stack, being one of the safest locations - * to clobber. + * This early in boot, there is one area of memory we know about with + * reasonable confidence that it isn't overlapped by Xen, and that's + * the Multiboot info structure in %ebx. Use it as a temporary stack. */ - mov $0x1000, %esp + + /* Preserve the field we're about to clobber. */ + mov (%ebx), %edx + lea 4(%ebx), %esp /* Calculate the load base address. */ call 1f 1: pop %esi sub $sym_offs(1b), %esi + /* Restore the clobbered field. */ + mov %edx, (%ebx) + /* Set up stack. */ lea STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp -- generated by git-patchbot for /home/xen/git/xen.git#staging
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