[PATCHv1 4/5] x86/viridian: set x87 FIP width to 4 for Windows guests

[David Vrabel <david.vrabel@citrix.com>]

Microsoft Windows always uses a 32-bit FPU state save/restore and expects
the FCS/FDS to be saved/restored.  Ensure that for these guests, the
hypervisor does 32-bit save/restore to preserve FCS/FDS.

These guests are identified by the write to the Guest OS ID MSR.

This fixes an 0x3D BugCheck when running the Driver Verifier in 64-bit
Windows.  This BugCheck occurs because a context switch would clear
FCS/FDS and Driver Verifier would assert because the FPU state changed.

We only set FIP width if it is still in auto-mode, to allow the toolstack
to override if necessary.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 xen/arch/x86/hvm/viridian.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/viridian.c b/xen/arch/x86/hvm/viridian.c
index 6bd844b..fb9f044 100644
--- a/xen/arch/x86/hvm/viridian.c
+++ b/xen/arch/x86/hvm/viridian.c
@@ -148,6 +148,30 @@ static void dump_guest_os_id(const struct domain *d)
            goi->fields.service_pack, goi->fields.build_number);
 }
 
+static void set_guest_os_id(struct domain *d, uint64_t val)
+{
+    const union viridian_guest_os_id *goi;
+
+    d->arch.hvm_domain.viridian.guest_os_id.raw = val;
+    goi = &d->arch.hvm_domain.viridian.guest_os_id;
+
+    /*
+     * Microsoft Windows only saves the lower 32-bits of FIP/FDP and
+     * can get upset if the selectors are not saved/restored by the
+     * hypervisor.
+     *
+     * Only do this if the FIP width is not in auto-mode, so this
+     * heuristic can be overriden by the toolstack.
+     */
+    if ( !d->arch.x87_fip_width )
+    {
+        if ( goi->fields.vendor == 1 && goi->fields.os == 4 )
+            d->arch.x87_fip_width = 4;
+    }
+
+    dump_guest_os_id(d);
+}
+
 static void dump_hypercall(const struct domain *d)
 {
     const union viridian_hypercall_gpa *hg;
@@ -334,8 +358,7 @@ int wrmsr_viridian_regs(uint32_t idx, uint64_t val)
     {
     case VIRIDIAN_MSR_GUEST_OS_ID:
         perfc_incr(mshv_wrmsr_osid);
-        d->arch.hvm_domain.viridian.guest_os_id.raw = val;
-        dump_guest_os_id(d);
+        set_guest_os_id(d, val);
         break;
 
     case VIRIDIAN_MSR_HYPERCALL:
-- 
2.1.4