Question about MSR_K7_HWCR in kvm_set_msr_common

[Jidong Xiao <jidong.xiao@xxxxxxxxx>]

Hi,

In kvm_set_msr_common(), I see that the follow piece of code will
handle the write operation to the register MSR_K7_HWCR.

case MSR_K7_HWCR:
          data &= ~(u64)0x40; /* ignore flush filter disable */
          data &= ~(u64)0x100; /* ignore ignne emulation enable */
          data &= ~(u64)0x8; /* ignore TLB cache disable */
          if (data != 0) {
               pr_unimpl(vcpu, "unimplemented HWCR wrmsr: 0x%llx\n",data);
               return 1;
           }
          break;

I am totally confused that, from this piece of code, we can see,
nothing will actually be written to MSR_K7_HWCR, if so, why do we
explicitly ignore some bits?

if we don't want the guest to write 0x40, 0x100, 0x8 to this register,
why don't we just return 1 and do nothing else. Like this:

case MSR_K7_HWCR:
         {
               pr_unimpl(vcpu, "unimplemented HWCR wrmsr: 0x%llx\n",data);
               return 1;
           }

Or, we can simply use the default case, which may also return 1.

So, my question is, if we explicitly emulate this register, why do we
also explicitly ignore all the write operation to this register?

-Jidong
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