Hi Christoffer,

On 24/01/2019 14:00, Christoffer Dall wrote:

Note that to avoid mapping the kvm_vmid_bits variable into hyp, we
simply forego the masking of the vmid value in kvm_get_vttbr and rely on
update_vmid to always assign a valid vmid value (within the supported
range).

[...]

-	kvm->arch.vmid = kvm_next_vmid;
+	vmid->vmid = kvm_next_vmid;
  	kvm_next_vmid++;
-	kvm_next_vmid &= (1 << kvm_vmid_bits) - 1;
-
-	/* update vttbr to be used with the new vmid */
-	pgd_phys = virt_to_phys(kvm->arch.pgd);
-	BUG_ON(pgd_phys & ~kvm_vttbr_baddr_mask(kvm));
-	vmid = ((u64)(kvm->arch.vmid) << VTTBR_VMID_SHIFT) & VTTBR_VMID_MASK(kvm_vmid_bits);
-	kvm->arch.vttbr = kvm_phys_to_vttbr(pgd_phys) | vmid | cnp;
+	kvm_next_vmid &= (1 << kvm_get_vmid_bits()) - 1;

The arm64 version of kvm_get_vmid_bits does not look cheap. Indeed it required 
to read the sanitized value of SYS_ID_AA64MMFR1_EL1 that is implemented using 
the function bsearch.

So wouldn't it be better to keep kvm_vmid_bits variable for use in update_vttbr()?

Cheers,

-- 
Julien Grall
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