[PATCH 10/10] x86/cpuid: Always enable faulting for the control domain

[Andrew Cooper <andrew.cooper3@citrix.com>]

The domain builder in libxc no longer depends on leaked CPUID information to
properly construct HVM domains.  Remove the control domain exclusion.

On capable hardware, this prevents all unintended leakage of hardware CPUID
values into the control domain, and brings the hypervisor leaves into view.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
---
 xen/arch/x86/cpu/intel.c | 18 +++---------------
 1 file changed, 3 insertions(+), 15 deletions(-)

diff --git a/xen/arch/x86/cpu/intel.c b/xen/arch/x86/cpu/intel.c
index 2e20327..238c47d 100644
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -159,22 +159,10 @@ static void intel_ctxt_switch_levelling(const struct vcpu *next)
 
 	if (cpu_has_cpuid_faulting) {
 		/*
-		 * We *should* be enabling faulting for the control domain.
-		 *
-		 * Unfortunately, the domain builder (having only ever been a
-		 * PV guest) expects to be able to see host cpuid state in a
-		 * native CPUID instruction, to correctly build a CPUID policy
-		 * for HVM guests (notably the xstate leaves).
-		 *
-		 * This logic is fundimentally broken for HVM toolstack
-		 * domains, and faulting causes PV guests to behave like HVM
-		 * guests from their point of view.
-		 *
-		 * Future development plans will move responsibility for
-		 * generating the maximum full cpuid policy into Xen, at which
-		 * this problem will disappear.
+		 * Always enable faulting for all PV domains.  Enable faulting
+		 * for HVM guests if they have configured it.
 		 */
-		set_cpuid_faulting(nextd && !is_control_domain(nextd) &&
+		set_cpuid_faulting(nextd &&
 				   (is_pv_domain(nextd) ||
 				    next->arch.cpuid_faulting));
 		return;
-- 
2.1.4


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