Re: [PATCH 06/14] KVM: arm/arm64: Factor out VMID into struct kvm_vmid

Julien Grall
2019-02-21T11:02:56Z

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