x86/pvh: Call C code via the kernel virtual mapping

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: 7243b93 ("xen/pvh: Bootstrap PVH guest")
Tested-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Message-ID: <20241009160438.3884381-8-ardb+git@google.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

Author: ardbiesheuvel

arch/x86/platform/pvh/head.S

@@ -172,7 +172,14 @@ 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
+	subq phys_base(%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.