linux-kernel - Re: [PATCH V1 3/6] xen/virtio: Add option to restrict memory access under Xen

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Date:   Sat, 23 Apr 2022 09:40:53 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Oleksandr Tyshchenko <olekstysh@...il.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Juergen Gross <jgross@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Julien Grall <julien@....org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH V1 3/6] xen/virtio: Add option to restrict memory access
 under Xen

Please split this into one patch that creates grant-dma-ops, and another
that sets up the virtio restricted access helpers.

> +
> +#ifdef CONFIG_ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
> +int arch_has_restricted_virtio_memory_access(void)
> +{
> +	return (xen_has_restricted_virtio_memory_access() ||
> +			cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT));
> +}

So instead of hardcoding Xen here, this seems like a candidate for
another cc_platform_has flag.

> +config XEN_VIRTIO
> +	bool "Xen virtio support"
> +	default n

n is the default default, so no need to specify it.

> +// SPDX-License-Identifier: GPL-2.0-only
> +/******************************************************************************

The all * line is not the usual kernel style, I'd suggest to drop it.

> +static struct page *xen_grant_dma_alloc_pages(struct device *dev, size_t size,
> +					      dma_addr_t *dma_handle,
> +					      enum dma_data_direction dir,
> +					      gfp_t gfp)
> +{
> +	WARN_ONCE(1, "xen_grant_dma_alloc_pages size %zu\n", size);
> +	return NULL;
> +}
> +
> +static void xen_grant_dma_free_pages(struct device *dev, size_t size,
> +				     struct page *vaddr, dma_addr_t dma_handle,
> +				     enum dma_data_direction dir)
> +{
> +	WARN_ONCE(1, "xen_grant_dma_free_pages size %zu\n", size);
> +}

Please just wire this up to the same implementation as .alloc and .free.

> +	spin_lock(&xen_grant_dma_lock);
> +	list_add(&data->list, &xen_grant_dma_devices);
> +	spin_unlock(&xen_grant_dma_lock);

Hmm, having to do this device lookup for every DMA operation is going
to suck. It might make sense to add a private field (e.g. as a union
with the iommu field) in struct device instead.

But if not you probably want to switch to a more efficient data
structure like the xarray at least.

> +EXPORT_SYMBOL_GPL(xen_grant_setup_dma_ops);

I don't think this has any modular users, or did I miss something?