On 02/05/2023 10:22 am, Roger Pau Monne wrote: > When booting the BSP the portion of the code executed from the > trampoline page will be using the GDT located in the hypervisor > .text.head section rather than the GDT located in the trampoline page.
It's more subtle than this. gdt_boot_descr references the trampoline GDT, but by it's position in the main Xen image. > > If skip_realmode is not set the GDT located in the trampoline page > will be loaded after having executed the BIOS call, otherwise the GDT > from .text.head will be used for all the protected mode trampoline > code execution. > > Note that both gdt_boot_descr and gdt_48 contain the same entries, but > the former is located inside the hypervisor .text section, while the > later lives in the relocated trampoline page. > > This is not harmful as-is, as both GDTs contain the same entries, but > for consistency with the APs switch the BSP trampoline code to also > use the GDT on the trampoline page. > > Signed-off-by: Roger Pau Monné <roger....@citrix.com> Reviewed-by: Andrew Cooper <andrew.coop...@citrix.com>, although ... > --- > xen/arch/x86/boot/trampoline.S | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S > index cdecf949b410..e4b4b9091d0c 100644 > --- a/xen/arch/x86/boot/trampoline.S > +++ b/xen/arch/x86/boot/trampoline.S > @@ -164,6 +164,12 @@ GLOBAL(trampoline_cpu_started) > > .code32 > trampoline_boot_cpu_entry: > + /* > + * Load the GDT from the relocated trampoline page rather than the > + * hypervisor .text section. > + */ > + lgdt bootsym_rel(gdt_48, 4) ... I'd suggest rewording this to simply /* Switch to trampoline GDT */, or perhaps with an "alias" in there somewhere. The important point here is that we want to shed all pre-trampoline state, and unexpectedly being on the wrong GDT alias certainly complicated debugging this... > + > cmpb $0,bootsym_rel(skip_realmode,5) > jnz .Lskip_realmode >
