linux-kernel - Re: [RFC PATCH 2/2] dma-direct: Fix dma_direct

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Date:   Thu, 7 Jul 2022 16:20:24 +0200
From:   Andrea Parri <parri.andrea@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Anvin <hpa@...or.com>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        linux-hyperv@...r.kernel.org, x86@...nel.org
Subject: Re: [RFC PATCH 2/2] dma-direct: Fix dma_direct_{alloc,free}() for
 Hyperv-V IVMs

> > @@ -305,6 +306,21 @@ void *dma_direct_alloc(struct device *dev, size_t size,
> >  		ret = page_address(page);
> >  		if (dma_set_decrypted(dev, ret, size))
> >  			goto out_free_pages;
> > +#ifdef CONFIG_HAS_IOMEM
> > +		/*
> > +		 * Remap the pages in the unencrypted physical address space
> > +		 * when dma_unencrypted_base is set (e.g., for Hyper-V AMD
> > +		 * SEV-SNP isolated guests).
> > +		 */
> > +		if (dma_unencrypted_base) {
> > +			phys_addr_t ret_pa = virt_to_phys(ret);
> > +
> > +			ret_pa += dma_unencrypted_base;
> > +			ret = memremap(ret_pa, size, MEMREMAP_WB);
> > +			if (!ret)
> > +				goto out_encrypt_pages;
> > +		}
> > +#endif
> 
> 
> So:
> 
> this needs to move into dma_set_decrypted, otherwise we don't handle
> the dma_alloc_pages case (never mind that this is pretty unreadable).
> 
> Which then again largely duplicates the code in swiotlb.  So I think
> what we need here is a low-level helper that does the
> set_memory_decrypted and memremap.  I'm not quite sure where it
> should go, but maybe some of the people involved with memory
> encryption might have good ideas.  unencrypted_base should go with
> it and then both swiotlb and dma-direct can call it.

Agreed, will look into this more  (other people's ideas welcome).


> > +	/*
> > +	 * If dma_unencrypted_base is set, the virtual address returned by
> > +	 * dma_direct_alloc() is in the vmalloc address range.
> > +	 */
> > +	if (!dma_unencrypted_base && is_vmalloc_addr(cpu_addr)) {
> >  		vunmap(cpu_addr);
> >  	} else {
> >  		if (IS_ENABLED(CONFIG_ARCH_HAS_DMA_CLEAR_UNCACHED))
> >  			arch_dma_clear_uncached(cpu_addr, size);
> > +#ifdef CONFIG_HAS_IOMEM
> > +		if (dma_unencrypted_base) {
> > +			memunmap(cpu_addr);
> > +			/* re-encrypt the pages using the original address */
> > +			cpu_addr = page_address(pfn_to_page(PHYS_PFN(
> > +					dma_to_phys(dev, dma_addr))));
> > +		}
> > +#endif
> >  		if (dma_set_encrypted(dev, cpu_addr, size))
> 
> Same on the unmap side.  It might also be worth looking into reordering
> the checks in some form instead o that raw dma_unencrypted_base check
> before the unmap.

Got it.

Thanks,
  Andrea