linux-kernel - RE: [PATCH] platform/x86/dell: add buffer allocation/free functions for SMI calls

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Date:   Fri, 18 Mar 2022 22:28:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Juergen Gross' <jgross@...e.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Dell.Client.Kernel@...l.com" <Dell.Client.Kernel@...l.com>
CC:     Stuart Hayes <stuart.w.hayes@...il.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <markgross@...nel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] platform/x86/dell: add buffer allocation/free functions
 for SMI calls

From: Juergen Gross
> Sent: 18 March 2022 16:56
> 
> On 18.03.22 16:22, David Laight wrote:
> > From: Juergen Gross
> >> Sent: 18 March 2022 15:10
> >>
> >> The dcdbas driver is used to call SMI handlers for both, dcdbas and
> >> dell-smbios-smm. Both drivers allocate a buffer for communicating
> >> with the SMI handler. The physical buffer address is then passed to
> >> the called SMI handler via %ebx.
> >>
> >> Unfortunately this doesn't work when running in Xen dom0, as the
> >> physical address obtained via virt_to_phys() is only a guest physical
> >> address, and not a machine physical address as needed by SMI.
> >
> > The physical address from virt_to_phy() is always wrong.
> > That is the physical address the cpu has for the memory.
> > What you want is the address the dma master interface needs to use.
> > That can be different for a physical system - no need for virtualisation.
> >
> > On x86 they do usually match, but anything with a full iommu
> > will need completely different addresses.
> 
> Yes, thanks for reminding me of that.
> 
> The SMI handler is running on the cpu, right? So using the DMA
> address is wrong in case of an IOMMU. I really need the machine
> physical address.

That ought to be handled by the 'dev' parameter to dma_alloc_coherent().

	David

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