topic MSR-Bitmaps in Software Archive

MSR-Bitmaps

heinerj — Fri, 13 Apr 2012 13:43:32 GMT

Having problems with the MSR bitmaps.
I read the IA32_VMX_PROCBASED_CTLS MSR, set bit 28 and stored it into the vmcs's Primary proc-based VM-execution controls field.
I then have the following structure in the vmcs header file:

struct MSR_BITMAP
{
u64 MSR_READ_LO[128];
u64 MSR_READ_HI[128];
u64 MSR_WRITE_LO[128];
u64 MSR_WRITE_HI[128];
} __attribute__ (( aligned (4096) ));

and in the vmcs.cI have the following code that should clear all bits in the MSR bitmap pointed to by the MSR_BITMAP address of the vmcs field so that no MSR read or write will cause a VM exit:

struct MSR_BITMAP MSR_BITMAP1;

unsigned long MSR_BITMAP_ADDR = (unsigned long) &MSR_BITMAP1;
memset(&MSR_BITMAP1, 0, sizeof(MSR_BITMAP1)); //this should clear all bits in the bitmap forcing no MSRs to cause a VM exit
__vmwrite (MSR_BITMAP_LO, (u32)MSR_BITMAP_ADDR);
__vmwrite (MSR_BITMAP_HI, (u32)(MSR_BITMAP_ADDR >> 32));

In the VM i have the following code trying to read a MSR:

rdmsrl(MSR_IA32_APIC_BASE_BSE, apic_base);

However when the VM tries to read the MSR I still get a VM-exit due to a MSR exception. How do I correctly setup the MSR bitmap of the VMCS so that I do not get a VM-exit caused by a MSR exception when I try to read a MSR in the VM. (Trying to write to an MSR in the VM causes a triple fault, hopefully solving this will fix that problem as well.)
Any suggestions?

MSR-Bitmaps

interruptrequestpack — Sat, 14 Apr 2012 11:15:55 GMT

Hello,

so if you have correctly set the use of MSR bitmaps in the execution control field maybe your RCX value is not correctly in his range.

What I mean is: if theMSR_READ_LO is in the range [0, 0x1FFF] so RCX (or ECX) value should be in the same range, the same for other kind of bitmaps. This happens because if you execute __rdmsr without a correct value (ie. in the range) in RCX a VM exit occurs.

EITHER if the bit in the "selected (low/high, read/write)" MSR bitmaps corresponding to the RCX value is 1. So the n_bit (with n = rcx_value) should not be 1.

Tell me what about this check, thanks

irp

MSR-Bitmaps

heinerj — Mon, 16 Apr 2012 19:04:08 GMT

Problem was with the privilege level of my CS segment. Was not able to view the MSRs because my privilege level was 3 not 0.