[PATCH 1/5] x86/pvh: Call C code via the kernel virtual mapping

[Ard Biesheuvel <ardb+git@xxxxxxxxxx>]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

Calling C code via a different mapping than it was linked at is
problematic, because the compiler assumes that RIP-relative and absolute
symbol references are interchangeable. GCC in particular may use
RIP-relative per-CPU variable references even when not using -fpic.

So call xen_prepare_pvh() via its kernel virtual mapping on x86_64, so
that those RIP-relative references produce the correct values. This
matches the pre-existing behavior for i386, which also invokes
xen_prepare_pvh() via the kernel virtual mapping before invoking
startup_32 with paging disabled again.

Fixes: 7243b93345f7 ("xen/pvh: Bootstrap PVH guest")
Tested-by: Jason Andryuk <jason.andryuk@xxxxxxx>
Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 arch/x86/platform/pvh/head.S | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index 64fca49cd88f..98ddd552885a 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -172,7 +172,13 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
        movq %rbp, %rbx
        subq $_pa(pvh_start_xen), %rbx
        movq %rbx, phys_base(%rip)
-       call xen_prepare_pvh
+
+       /* Call xen_prepare_pvh() via the kernel virtual mapping */
+       leaq xen_prepare_pvh(%rip), %rax
+       addq $__START_KERNEL_map, %rax
+       ANNOTATE_RETPOLINE_SAFE
+       call *%rax
+
        /*
         * Clear phys_base.  __startup_64 will *add* to its value,
         * so reset to 0.
-- 
2.46.0.792.g87dc391469-goog