x86: handle paged gfn in wrmsr_hypervisor_regs

If xenpaging is started very early for a guest the gfn for the hypercall
page may be paged-out already. This leads to a guest crash:

...
(XEN) HVM10: Allocated Xen hypercall page at 169ff000
(XEN) traps.c:654:d10 Bad GMFN 169ff (MFN 3e900000000) to MSR 40000000
(XEN) HVM10: Detected Xen v4.3
(XEN) io.c:201:d10 MMIO emulation failed @ 0008:c2c2c2c2: 18 7c 55 6d 03 83 ff ff 10 7c
(XEN) hvm.c:1253:d10 Triple fault on VCPU0 - invoking HVM shutdown action 1.
(XEN) HVM11: HVM Loader
...

Update return codes of wrmsr_hypervisor_regs, update callers to deal
with the new return codes:
 0: not handled
 1: handled
 -EAGAIN: retry

Currently wrmsr_hypervisor_regs will not return the following error, it
will be added in a separate patch:
 -EINVAL: error during handling

Also update the gdprintk to handle a page value of NULL to avoid
printing a bogus MFN value. Update also computing of MSR value in
gdprintk, the idx was always zero.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Keir Fraser <keir@xen.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 51187a932a..d0de44a2ae 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2088,7 +2088,16 @@ static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content)
             case HNDL_unhandled:
                 if ( (vmx_write_guest_msr(msr, msr_content) != 0) &&
                      !is_last_branch_msr(msr) )
-                    wrmsr_hypervisor_regs(msr, msr_content);
+                    switch ( wrmsr_hypervisor_regs(msr, msr_content) )
+                    {
+                    case -EAGAIN:
+                        return X86EMUL_RETRY;
+                    case 0:
+                    case 1:
+                        break;
+                    default:
+                        goto gp_fault;
+                    }
                 break;
             case HNDL_exception_raised:
                 return X86EMUL_EXCEPTION;