[PATCH] x86/PV: conditionally avoid raising #GP for early guest MSR accesses

[Jan Beulich <jbeulich@xxxxxxxx>]

Prior to 4.15 Linux, when running in PV mode, did not install a #GP
handler early enough to cover for example the rdmsrl_safe() of
MSR_K8_TSEG_ADDR in bsp_init_amd() (not to speak of the unguarded read
of MSR_K7_HWCR later in the same function). The respective change
(42b3a4cb5609 "x86/xen: Support early interrupts in xen pv guests") was
backported to 4.14, but no further - presumably since it wasn't really
easy because of other dependencies.

Therefore, to prevent our change in the handling of guest MSR accesses
to render PV Linux 4.13 and older unusable on at least AMD systems, make
the raising of #GP on these paths conditional upon the guest having
installed a handler. Producing zero for reads and discarding writes
isn't necessarily correct and may trip code trying to detect presence of
MSRs early, but since such detection logic won't work without a #GP
handler anyway, this ought to be a fair workaround.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -886,7 +886,7 @@ static int read_msr(unsigned int reg, ui
         if ( ret == X86EMUL_EXCEPTION )
             x86_emul_hw_exception(TRAP_gp_fault, 0, ctxt);
 
-        return ret;
+        goto done;
     }
 
     switch ( reg )
@@ -999,7 +999,16 @@ static int read_msr(unsigned int reg, ui
         return X86EMUL_OKAY;
     }
 
-    return X86EMUL_UNHANDLEABLE;
+ done:
+    if ( ret != X86EMUL_OKAY && !curr->arch.pv.trap_ctxt[X86_EXC_GP].address )
+    {
+        gprintk(XENLOG_WARNING, "faking RDMSR 0x%08x\n", reg);
+        *val = 0;
+        x86_emul_reset_event(ctxt);
+        ret = X86EMUL_OKAY;
+    }
+
+    return ret;
 }
 
 static int write_msr(unsigned int reg, uint64_t val,
@@ -1016,7 +1025,7 @@ static int write_msr(unsigned int reg, u
         if ( ret == X86EMUL_EXCEPTION )
             x86_emul_hw_exception(TRAP_gp_fault, 0, ctxt);
 
-        return ret;
+        goto done;
     }
 
     switch ( reg )
@@ -1172,7 +1181,15 @@ static int write_msr(unsigned int reg, u
         return X86EMUL_OKAY;
     }
 
-    return X86EMUL_UNHANDLEABLE;
+ done:
+    if ( ret != X86EMUL_OKAY && !curr->arch.pv.trap_ctxt[X86_EXC_GP].address )
+    {
+        gprintk(XENLOG_WARNING, "dropping WRMSR 0x%08x\n", reg);
+        x86_emul_reset_event(ctxt);
+        ret = X86EMUL_OKAY;
+    }
+
+    return ret;
 }
 
 static int cache_op(enum x86emul_cache_op op, enum x86_segment seg,