Re: [RFC PATCH 12/15] KVM: x86/mmu: Split large pages when dirty logging is enabled

[Peter Xu <peterx@redhat.com>]

Hi, David,

On Fri, Nov 19, 2021 at 11:57:56PM +0000, David Matlack wrote:
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 2a7564703ea6..432a4df817ec 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1232,6 +1232,9 @@ struct kvm_arch {
>  	hpa_t	hv_root_tdp;
>  	spinlock_t hv_root_tdp_lock;
>  #endif
> +
> +	/* MMU caches used when splitting large pages during VM-ioctls. */
> +	struct kvm_mmu_memory_caches split_caches;

Are mmu_gfn_array_cache and mmu_pte_list_desc_cache wasted here?  I saw that
"struct kvm_mmu_memory_cache" still takes up quite a few hundreds of bytes,
just want to make sure we won't waste them in vain.

[...]

> +int mmu_topup_split_caches(struct kvm *kvm)
> +{
> +	struct kvm_mmu_memory_caches *split_caches = &kvm->arch.split_caches;
> +	int r;
> +
> +	assert_split_caches_invariants(kvm);
> +
> +	r = kvm_mmu_topup_memory_cache(&split_caches->page_header_cache, 1);
> +	if (r)
> +		goto out;
> +
> +	r = kvm_mmu_topup_memory_cache(&split_caches->shadow_page_cache, 1);
> +	if (r)
> +		goto out;

Is it intended to only top-up with one cache object?  IIUC this means we'll try
to proactively yield the cpu for each of the huge page split right after the
object is consumed.

Wondering whether it be more efficient to make it a slightly larger number, so
we don't overload the memory but also make the loop a bit more efficient.

> +
> +	return 0;
> +
> +out:
> +	pr_warn("Failed to top-up split caches. Will not split large pages.\n");
> +	return r;
> +}

Thanks,

-- 
Peter Xu