[PATCH v2] KVM: x86/pt: Ignore all unknown Intel PT capabilities

[Like Xu]

From: Like Xu <likexu@xxxxxxxxxxx>

Some of the new Intel PT capabilities (e.g. SDM Vol3, 32.2.4 Event
Tracing, it exposes details about the asynchronous events, when they are
generated, and when their corresponding software event handler completes
execution) cannot be safely and fully emulated by the KVM, especially
emulating the simultaneous writing of guest PT packets generated by
the KVM to the guest PT buffer.

For KVM, it's better to advertise currently supported features based on
the "static struct pt_cap_desc" implemented in the host PT driver and
ignore _all_ unknown features before they have been investigated one by
one and supported in a safe manner, leaving the rest as system-wide-only
tracing capabilities.

Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
v1 -> v2 Changelog:
- Be safe and ignore _all_ unknown capabilities. (Paolo)

Previous:
https://lore.kernel.org/kvm/20220106085533.84356-1-likexu@xxxxxxxxxxx/

 arch/x86/kvm/cpuid.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 0b920e12bb6d..439b93359848 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -901,6 +901,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 			break;
 		}
 
+		/* It's better to be safe and ignore _all_ unknown capabilities. */
+		entry->ebx &= GENMASK(5, 0);
 		for (i = 1, max_idx = entry->eax; i <= max_idx; ++i) {
 			if (!do_host_cpuid(array, function, i))
 				goto out;
-- 
2.33.1