linux-kernel - Re: [PATCH] x86/PCI: limit the size of the 64bit BAR to 256GB

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Message-ID: <ca0ac4ea-59cd-375c-f0ef-a80b2eb6deb0@gmail.com>
Date:   Tue, 12 Dec 2017 19:38:51 +0100
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH] x86/PCI: limit the size of the 64bit BAR to 256GB

Am 12.12.2017 um 19:12 schrieb Bjorn Helgaas:
> [+cc Boris, Juergen, xen-devel]
>
> On Mon, Dec 11, 2017 at 04:04:52PM +0100, Christian König wrote:
>> Xen hides a bit of system memory from the OS for its own purpose by
>> intercepting e820. This memory is unfortunately not reported as
>> reserved, but rather completely invisible.
>>
>> Avoid this address space collision and possible similar problems by
>> limiting the size of the newly allocated root hub window to 256GB which
>> should be sufficient for the short term.
> It sounds like Boris is working on a more complete fix, so I'm going
> to drop this for now.  This changelog includes a few more details, but
> I think it makes implicit assumptions about where memory and holes
> are and how big they are, and it's still not clear why 256GB is the
> right number.  Is it connected to the expected size of the BAR, or
> related somehow to the size of the hole?

256GB was just an arbitrary number I've thought should work for at least 
my use case.

And yes Boris is working on a more complete and cleaner fix. I agree 
that we can completely drop my patch for now.

> If we need this as a short-term workaround, we can do that, but I
> would like to include a reference to f5775e0b6116 ("x86/xen: discard
> RAM regions above the maximum reservation") and somehow make what's
> going on here a little more explicit.

That patch looks more like it only applies to Xen guests, but not dom0. 
But take that with a grain of salt I really don't know anything about 
that code.

Christian.

>
>> Signed-off-by: Christian König <christian.koenig@....com>
>> ---
>>   arch/x86/pci/fixup.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/pci/fixup.c b/arch/x86/pci/fixup.c
>> index 8f86060f5cf6..ed8bc6ab0573 100644
>> --- a/arch/x86/pci/fixup.c
>> +++ b/arch/x86/pci/fixup.c
>> @@ -702,7 +702,7 @@ static void pci_amd_enable_64bit_bar(struct pci_dev *dev)
>>   	res->name = "PCI Bus 0000:00";
>>   	res->flags = IORESOURCE_PREFETCH | IORESOURCE_MEM |
>>   		IORESOURCE_MEM_64 | IORESOURCE_WINDOW;
>> -	res->start = 0x100000000ull;
>> +	res->start = 0xbd00000000ull;
>>   	res->end = 0xfd00000000ull - 1;
>>   
>>   	/* Just grab the free area behind system memory for this */
>> -- 
>> 2.11.0
>>