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

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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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