On Thu, 21 Feb 2019 11:02:56 +0000
Julien Grall [off-list ref] wrote:

Hi Julien,

Hi Christoffer,

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

quoted

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).  

[...]

quoted

-	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()?

How often does this happen? Can you measure this overhead at all?

My understanding is that we hit this path on rollover only, having IPIed
all CPUs and invalidated all TLBs. I seriously doubt you can observe
any sort of overhead at all, given that it is so incredibly rare. But
feel free to prove me wrong!

Thanks,

	M.
-- 
Without deviation from the norm, progress is not possible.

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