linux-kernel - Re: [PATCH] vfio/iommu_type1: report the IOMMU aperture info

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Date:   Thu, 30 Nov 2017 12:57:56 +0000
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Pierre Morel <pmorel@...ux.vnet.ibm.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>
Cc:     "cohuck@...hat.com" <cohuck@...hat.com>,
        "borntraeger@...ibm.com" <borntraeger@...ibm.com>,
        "zyimin@...ux.vnet.ibm.com" <zyimin@...ux.vnet.ibm.com>,
        "pasic@...ux.vnet.ibm.com" <pasic@...ux.vnet.ibm.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vfio/iommu_type1: report the IOMMU aperture info

Hello,

On 30/11/17 11:34, Pierre Morel wrote:
[...]
> +/**
> + * vfio_get_aperture - report minimal aperture of a vfio_iommu
> + * @iommu: the current vfio_iommu
> + * @start: a pointer to the aperture start
> + * @end  : a pointer to the aperture end
> + *
> + * This function iterate on the domains using the given vfio_iommu
> + * and restrict the aperture to the minimal aperture common
> + * to all domains sharing this vfio_iommu.
> + */
> +static void vfio_get_aperture(struct vfio_iommu *iommu, uint64_t *start,
> +				uint64_t *end)
> +{
> +	struct iommu_domain_geometry geometry;
> +	struct vfio_domain *domain;
> +
> +	*start = 0;
> +	*end = U64_MAX;

I wonder if the default values should also reflect what the VFIO
implementation actually supports. Looking at vfio_dma_do_map, a 32-bit
host will reject any iova greater than 32 bits. In addition,
vfio_dma_do_unmap doesn't support unmapping the last page of a 64-bit
address space (existing IOMMUs would probably reject map requests with
IOVA > 52 bits anyway, but if they don't report a domain aperture, VFIO
can't guess it).

I think it's convenient to use VFIO_IOMMU_UNMAP_DMA on the full address
space when an unmap-all is needed, maybe we could provide default aperture
values that help doing this? (~0U for 32-bit and (~0ULL - PAGE_SIZE) for
64-bit)

Thanks,
Jean