linux-kernel - Re: [PATCH v4 11/15] pci: Add pci_iomap_shared{,

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Date:   Tue, 24 Aug 2021 08:07:55 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Richard Henderson <rth@...ddle.net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        James E J Bottomley <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        "David S . Miller" <davem@...emloft.net>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 11/15] pci: Add pci_iomap_shared{,_range}

On Mon, Aug 23, 2021 at 05:30:54PM -0700, Kuppuswamy, Sathyanarayanan wrote:
> 
> 
> On 8/23/21 4:56 PM, Michael S. Tsirkin wrote:
> > > Add a new variant of pci_iomap for mapping all PCI resources
> > > of a devices as shared memory with a hypervisor in a confidential
> > > guest.
> > > 
> > > Signed-off-by: Andi Kleen<ak@...ux.intel.com>
> > > Signed-off-by: Kuppuswamy Sathyanarayanan<sathyanarayanan.kuppuswamy@...ux.intel.com>
> > I'm a bit puzzled by this part. So why should the guest*not*  map
> > pci memory as shared? And if the answer is never (as it seems to be)
> > then why not just make regular pci_iomap DTRT?
> 
> It is in the context of confidential guest (where VMM is un-trusted). So
> we don't want to make all PCI resource as shared. It should be allowed
> only for hardened drivers/devices.

Well, assuming the host can do any damage when mapped shared that also
means not mapping it shared will completely break the drivers.