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SubjectRe: [PATCH RFC 1/2] KVM: x86: allocate vcpu->arch.cpuid_entries dynamically
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@redhat.com>
> ---

...

> @@ -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
>

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