RE: [PATCH V2] arm: xen: mm: use __GPF_DMA32 for arm64

From: Peng Fan <peng.fan@nxp.com>
To: Stefano Stabellini <sstabellini@kernel.org>,
	Christoph Hellwig <hch@infradead.org>
Cc: "jgross@suse.com" <jgross@suse.com>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	"linux@armlinux.org.uk" <linux@armlinux.org.uk>,
	Julien Grall <Julien.Grall@arm.com>,
	dl-linux-imx <linux-imx@nxp.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"boris.ostrovsky@oracle.com" <boris.ostrovsky@oracle.com>,
	nd <nd@arm.com>, "will@kernel.org" <will@kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	Robin Murphy <Robin.Murphy@arm.com>
Subject: RE: [PATCH V2] arm: xen: mm: use __GPF_DMA32 for arm64
Date: Wed, 11 Sep 2019 02:06:52 +0000	[thread overview]
Message-ID: <AM0PR04MB448166E527EF4ABFCE3DD59688B10@AM0PR04MB4481.eurprd04.prod.outlook.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1908301926500.21347@sstabellini-ThinkPad-T480s>

> Subject: Re: [PATCH V2] arm: xen: mm: use __GPF_DMA32 for arm64
> 
> + Juergen, Boris
> 
> On Fri, 30 Aug 2019, Christoph Hellwig wrote:
> > Can we take a step back and figure out what we want to do here?
> >
> > AFAICS this function allocates memory for the swiotlb-xen buffer, and
> > that means it must be <= 32-bit addressable to satisfy the DMA API
> > guarantees.  That means we generally want to use GFP_DMA32
> everywhere
> > that exists, but on systems with odd zones we might want to dip into
> > GFP_DMA.  This also means swiotlb-xen doesn't actually do the right
> > thing on x86 at the moment.  So shouldn't we just have one common
> > routine in swiotlb-xen.c that checks if we have CONFIG_ZONE_DMA32 set,
> > then try GFP_DMA32, and if not check if CONFIG_ZONE_DMA is set and
> > then try that, else default to GFP_KERNEL?
> 
> Yes, for ARM/ARM64 it makes a lot of sense given that dom0 is 1:1 mapped
> (pseudo-physical == physical).  I'll let Juergen and Boris comment on the x86
> side of things, but on x86 PV Dom0 is not 1:1 mapped so
> GFP_DMA32 is probably not meaningful.

If we only take ARM/ARM64, so could the following patch be ok?

diff --git a/arch/arm/xen/mm.c b/arch/arm/xen/mm.c
index d33b77e9add3..e5a6a73b2e06 100644
--- a/arch/arm/xen/mm.c
+++ b/arch/arm/xen/mm.c
@@ -28,7 +28,11 @@ unsigned long xen_get_swiotlb_free_pages(unsigned int order)

        for_each_memblock(memory, reg) {
                if (reg->base < (phys_addr_t)0xffffffff) {
+#ifdef CONFIG_ZONE_DMA32
+                       flags |= __GFP_DMA32;
+#else
                        flags |= __GFP_DMA;
+#endif
                        break;
                }
        }

Thanks,
Peng.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
