Re: [PATCH RFC 1/2] KVM: x86: allocate vcpu->arch.cpuid_entries dynamically

[Sean Christopherson]

On Tue, Sep 15, 2020 at 05:43:05PM +0200, Vitaly Kuznetsov wrote:
> The current limit for guest CPUID leaves (KVM_MAX_CPUID_ENTRIES, 80)
> is reported to be insufficient but before we bump it let's switch to
> allocating vcpu->arch.cpuid_entries dynamically. Currenly,

                                                   Currently,

> 'struct kvm_cpuid_entry2' is 40 bytes so vcpu->arch.cpuid_entries is
> 3200 bytes which accounts for 1/4 of the whole 'struct kvm_vcpu_arch'
> but having it pre-allocated (for all vCPUs which we also pre-allocate)
> gives us no benefits.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---

...

> @@ -241,18 +253,31 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
>  			      struct kvm_cpuid2 *cpuid,
>  			      struct kvm_cpuid_entry2 __user *entries)
>  {
> +	struct kvm_cpuid_entry2 *cpuid_entries2 = NULL;
>  	int r;
>  
>  	r = -E2BIG;
>  	if (cpuid->nent > KVM_MAX_CPUID_ENTRIES)
>  		goto out;
>  	r = -EFAULT;
> -	if (copy_from_user(&vcpu->arch.cpuid_entries, entries,
> -			   cpuid->nent * sizeof(struct kvm_cpuid_entry2)))
> -		goto out;
> +
> +	if (cpuid->nent) {
> +		cpuid_entries2 = vmemdup_user(entries,
> +					      array_size(sizeof(cpuid_entries2[0]),
> +							 cpuid->nent));

Any objection to using something super short like "e2" instead of cpuid_entries2
so that this can squeeze on a single line, or at least be a little more sane?

> +		if (IS_ERR(cpuid_entries2)) {
> +			r = PTR_ERR(cpuid_entries2);
> +			goto out;

Don't suppose you'd want to opportunistically kill off these gotos?

> +		}
> +	}
> +	kvfree(vcpu->arch.cpuid_entries);

This is a bit odd.  The previous vcpu->arch.cpuid_entries is preserved on
allocation failure, but not on kvm_check_cpuid() failure.  Being destructive
on the "check" failure was always a bit odd, but it really stands out now.

Given that kvm_check_cpuid() now only does an actual check and not a big
pile of updates, what if we refactored the guts of kvm_find_cpuid_entry()
into yet another helper so that kvm_check_cpuid() could check the input
before crushing vcpu->arch.cpuid_entries?

	if (cpuid->nent) {
		e2 = vmemdup_user(entries, array_size(sizeof(e2[0]), cpuid->nent));
		if (IS_ERR(e2))
			return PTR_ERR(e2);

		r = kvm_check_cpuid(e2, cpuid->nent);
		if (r)
			return r;
	}

	vcpu->arch.cpuid_entries = e2;
	vcpu->arch.cpuid_nent = cpuid->nent;
	return 0;

> +	vcpu->arch.cpuid_entries = cpuid_entries2;
>  	vcpu->arch.cpuid_nent = cpuid->nent;
> +
>  	r = kvm_check_cpuid(vcpu);
>  	if (r) {
> +		kvfree(vcpu->arch.cpuid_entries);
> +		vcpu->arch.cpuid_entries = NULL;
>  		vcpu->arch.cpuid_nent = 0;
>  		goto out;
>  	}
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1994602a0851..42259a6ec1d8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9610,6 +9610,7 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
>  	kvm_mmu_destroy(vcpu);
>  	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>  	free_page((unsigned long)vcpu->arch.pio_data);
> +	kvfree(vcpu->arch.cpuid_entries);
>  	if (!lapic_in_kernel(vcpu))
>  		static_key_slow_dec(&kvm_no_apic_vcpu);
>  }
> -- 
> 2.25.4
>