[PATCH v3 8/9] target/i386/mshv: filter out CET bits in cpuid

[Magnus Kulke <magnuskulke@linux.microsoft.com>]

e46dcef1d0 introduced CET_U/CET_S xstate bits in the ExtSaveArea. The
response in EBX of a 0x4,1 cpuid query will contain a size contingent on
what the guest enabled in the IA32_XSS MSR (compacted xsave size).

Since the MSHV accelerator currently responds with static answers to
0x4d cpuid queries, we'll have to disable those bits, even if the host
supports them.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
---
 target/i386/mshv/mshv-cpu.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/target/i386/mshv/mshv-cpu.c b/target/i386/mshv/mshv-cpu.c
index ee25eb5f6f..2c66a52709 100644
--- a/target/i386/mshv/mshv-cpu.c
+++ b/target/i386/mshv/mshv-cpu.c
@@ -1663,6 +1663,25 @@ uint32_t mshv_get_supported_cpuid(uint32_t func, uint32_t idx, int reg)
     if (func == 0x01       && reg == R_ECX) {
         ret &= ~CPUID_EXT_VMX;
     }
+
+    /*
+     * MSHV currently uses static CPUID intercept results for leaf 0xD.
+     * However there are feature-responses that are dynamic based on what a
+     * guest enables in XCR0 and XSS, such as CET shadow stack.
+     *
+     * A guest which doesn't know about those features yet would encounter an
+     * unexpcted CPUID[0xD,1].EBX (compactes XSAVE size) and either fail
+     * or gracefully degrade by not using XSAVE at all.
+     *
+     * To avoid this, we filter out supervisor xstate features.
+     */
+    if (func == 0x07 && idx == 0 && reg == R_ECX) {
+        ret &= ~CPUID_7_0_ECX_CET_SHSTK;
+    }
+    if (func == 0x07 && idx == 0 && reg == R_EDX) {
+        ret &= ~CPUID_7_0_EDX_CET_IBT;
+    }
+
     return ret;
 }
 
-- 
2.34.1