linux-kernel - Re: [PATCH -tip/apic 2/2] x86/MSI: Allocate as many multiple-MSIs as requested

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Date:	Mon, 29 Apr 2013 08:22:09 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Alexander Gordeev" <agordeev@...hat.com>
Cc:	"Joerg Roedel" <joro@...tes.org>,
	"Bjorn Helgaas" <bhelgaas@...gle.com>,
	"Suresh Siddha" <suresh.b.siddha@...el.com>, <x86@...nel.org>,
	"Yinghai Lu" <yinghai@...nel.org>,
	"Ingo Molnar" <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	<linux-pci@...r.kernel.org>
Subject: Re: [PATCH -tip/apic 2/2] x86/MSI: Allocate as many
 multiple-MSIs as requested

>>> On 29.04.13 at 06:33, Alexander Gordeev <agordeev@...hat.com> wrote:
> --- a/drivers/iommu/irq_remapping.c
> +++ b/drivers/iommu/irq_remapping.c
> @@ -55,19 +55,19 @@ static int do_setup_msi_irqs(struct pci_dev *dev, int nvec)
>  	unsigned int irq;
>  	struct msi_desc *msidesc;
>  
> -	nvec = __roundup_pow_of_two(nvec);
> -
>  	WARN_ON(!list_is_singular(&dev->msi_list));
>  	msidesc = list_entry(dev->msi_list.next, struct msi_desc, list);
>  	WARN_ON(msidesc->irq);
>  	WARN_ON(msidesc->msi_attrib.multiple);
> +	WARN_ON(msidesc->nvec);
>  
>  	node = dev_to_node(&dev->dev);
>  	irq = __create_irqs(get_nr_irqs_gsi(), nvec, node);
>  	if (irq == 0)
>  		return -ENOSPC;
>  
> -	msidesc->msi_attrib.multiple = ilog2(nvec);
> +	msidesc->nvec = nvec;
> +	msidesc->msi_attrib.multiple = ilog2(__roundup_pow_of_two(nvec));
>  	for (sub_handle = 0; sub_handle < nvec; sub_handle++) {
>  		if (!sub_handle) {
>  			index = msi_alloc_remapped_irq(dev, irq, nvec);

This breaks the interface to IOMMU-specific code: While Intel's
implementation does bump the number of allocated IRTEs to a
power of 2, AMD's doesn't, and hence the tail entries in the block
that don't get allocated here can get used for another device,
thus creating a security hole when both devices aren't owned by
the same guest (with the host being considered a special kind of
guest for this purpose).

IOW, while you can conserve on the number of vectors allocated,
you can't on the IRTEs, and I think this should be taken care of in
the generic IOMMU code, not in the individual vendor
implementations.

Jan

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