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devvmx: skip tsc offset writing if CPU doesn't support it

--- a/sys/src/9/pc/devvmx.c

+++ b/sys/src/9/pc/devvmx.c

@@ -267,6 +267,7 @@

 	char errstr[ERRMAX];

 	Ureg ureg;

 	uvlong tscoffset;

+	u32int procbctls;

 	uintptr cr2;

 	uintptr xcr0;

 	uintptr dr[8]; /* DR7 is also kept in VMCS */

@@ -962,6 +963,7 @@

 	x |= PROCB_USECTLS2;

 	x &= msr >> 32;

 	vmcswrite(PROCB_CTLS, x);

+	vmx->procbctls = x;

 	if(rdmsr(VMX_PROCB_CTLS2_MSR, &msr) < 0) error("rdmsr(VMX_PROCB_CTLS2_MSR failed");

 	x = PROCB_EPT | PROCB_VPID | PROCB_UNRESTR;

@@ -1062,10 +1064,12 @@

 	vmcswrite(VMEXIT_MSRSTADDR, PADDR(vmx->msrguest));

 	vmcswrite(VMEXIT_MSRLDADDR, PADDR(vmx->msrhost));

 	vmcswrite(MSR_BITMAP, PADDR(vmx->msrbits));

-	

-	cycles(&vmx->tscoffset);

-	vmcswrite(VMCS_TSC_OFFSET, vmx->tscoffset);

+	if(vmx->procbctls & PROCB_TSCOFFSET){

+		cycles(&vmx->tscoffset);

+		vmcswrite(VMCS_TSC_OFFSET, vmx->tscoffset);

+	}

+

 	if(sizeof(uintptr) == 8){

 		vmxaddmsr(vmx, Star, 0);

 		vmxaddmsr(vmx, Lstar, 0);

@@ -1682,7 +1686,7 @@

 			}

 			rc = vmlaunch(&vmx->ureg, vmx->launched);

 			cycles(&end);

-			useend = 1;

+			useend = vmx->procbctls & PROCB_TSCOFFSET;

 			vmx->cr2 = getcr2();

 			if(m->xcr0 != 0 && vmx->xcr0 != m->xcr0)

 				putxcr0(m->xcr0);