linux-kernel - Re: [PATCH 09/11] KVM: guest_memfd: Add interface for populating gmem pages with user data

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Message-ID: <ZimIfFUMPmF_dV-V@google.com>
Date: Wed, 24 Apr 2024 15:32:28 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, michael.roth@....com, 
	isaku.yamahata@...el.com
Subject: Re: [PATCH 09/11] KVM: guest_memfd: Add interface for populating gmem
 pages with user data

On Thu, Apr 04, 2024, Paolo Bonzini wrote:
> +long kvm_gmem_populate(struct kvm *kvm, gfn_t gfn, void __user *src, long npages,
> +		       int (*post_populate)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> +					    void __user *src, int order, void *opaque),

Add a typedef for callback?  If only to make this prototype readable.

> +long kvm_gmem_populate(struct kvm *kvm, gfn_t gfn, void __user *src, long npages,
> +		       int (*post_populate)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> +					    void __user *src, int order, void *opaque),
> +		       void *opaque)
> +{
> +	struct file *file;
> +	struct kvm_memory_slot *slot;
> +
> +	int ret = 0, max_order;
> +	long i;
> +
> +	lockdep_assert_held(&kvm->slots_lock);
> +	if (npages < 0)
> +		return -EINVAL;
> +
> +	slot = gfn_to_memslot(kvm, gfn);
> +	if (!kvm_slot_can_be_private(slot))
> +		return -EINVAL;
> +
> +	file = kvm_gmem_get_file(slot);
> +	if (!file)
> +		return -EFAULT;
> +
> +	filemap_invalidate_lock(file->f_mapping);
> +
> +	npages = min_t(ulong, slot->npages - (gfn - slot->base_gfn), npages);
> +	for (i = 0; i < npages; i += (1 << max_order)) {
> +		gfn_t this_gfn = gfn + i;

KVM usually does something like "start_gfn" or "base_gfn", and then uses "gfn"
for the one gfn that's being processed.  And IMO that's much better because the
propotype for kvm_gmem_populate() does not make it obvious that @gfn is the base
of a range, not a singular gfn to process.


> +		kvm_pfn_t pfn;
> +
> +		ret = __kvm_gmem_get_pfn(file, slot, this_gfn, &pfn, &max_order, false);
> +		if (ret)
> +			break;
> +
> +		if (!IS_ALIGNED(this_gfn, (1 << max_order)) ||
> +		    (npages - i) < (1 << max_order))
> +			max_order = 0;
> +
> +		if (post_populate) {

Is there any use for this without @post_populate?  I.e. why make this optional?

> +			void __user *p = src ? src + i * PAGE_SIZE : NULL;

Eh, I would vote to either define "p" at the top of the loop.  

> +			ret = post_populate(kvm, this_gfn, pfn, p, max_order, opaque);
> +		}
> +
> +		put_page(pfn_to_page(pfn));
> +		if (ret) {
> +			/*
> +			 * Punch a hole so that FGP_CREAT_ONLY can succeed
> +			 * again.
> +			 */
> +			kvm_gmem_undo_get_pfn(file, slot, this_gfn, max_order);
> +			break;
> +		}
> +	}
> +
> +	filemap_invalidate_unlock(file->f_mapping);
> +
> +	fput(file);
> +	return ret && !i ? ret : i;
> +}
> +EXPORT_SYMBOL_GPL(kvm_gmem_populate);
> -- 
> 2.43.0
> 
>