KVM: selftests: Enable checking on xcomp_bv in amx_test

After tilerelease instruction, AMX tiles are in INIT state. According to
Intel SDM vol 1. 13.10: "If RFBM[i] = 1, XSTATE_BV[i] is set to the
value of XINUSE[i].", XSTATE_BV[18] should be cleared after xsavec.

On the other hand, according to Intel SDM vol 1. 13.4.3: "If XCOMP_BV[i] =
1, state component i is located at a byte offset locationI from the base
address of the XSAVE area". Since at the time of xsavec, XCR0[18] is set
indicating AMX tile data component is still enabled, xcomp_bv[18] should be
set.

Complete the checks by adding the assert to xcomp_bv[18] after xsavec.

Signed-off-by: Mingwei Zhang <mizhang@google.com>
Link: https://lore.kernel.org/r/20230221163655.920289-5-mizhang@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: mzhang3579

tools/testing/selftests/kvm/x86_64/amx_test.c

@@ -197,6 +197,7 @@ static void __attribute__((__flatten__)) guest_code(struct tile_config *amx_cfg,
 	xstate->header.xstate_bv = XFEATURE_MASK_XTILEDATA;
 	__xsavec(xstate, XFEATURE_MASK_XTILEDATA);
 	GUEST_ASSERT(!(xstate->header.xstate_bv & XFEATURE_MASK_XTILEDATA));
+	GUEST_ASSERT(xstate->header.xcomp_bv & XFEATURE_MASK_XTILEDATA);
 
 	/* xfd=0x40000, disable amx tiledata */
 	wrmsr(MSR_IA32_XFD, XFEATURE_MASK_XTILEDATA);