linux-kernel - Re: [PATCH v8 6/6] PCI: Fix bug resulting in double hpmemsize being assigned to MMIO window

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Message-ID: <20191008122710.GK2819@lahna.fi.intel.com>
Date:   Tue, 8 Oct 2019 15:27:10 +0300
From:   "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>
To:     Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "corbet@....net" <corbet@....net>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "logang@...tatee.com" <logang@...tatee.com>
Subject: Re: [PATCH v8 6/6] PCI: Fix bug resulting in double hpmemsize being
 assigned to MMIO window

On Fri, Jul 26, 2019 at 12:55:03PM +0000, Nicholas Johnson wrote:
> Background
> ==========================================================================

I don't think the above are needed.

> Currently, the kernel can sometimes assign the MMIO_PREF window
> additional size into the MMIO window, resulting in double the MMIO
> additional size, even if the MMIO_PREF window was successful.
> 
> This happens if in the first pass, the MMIO_PREF succeeds but the MMIO
> fails. In the next pass, because MMIO_PREF is already assigned, the
> attempt to assign MMIO_PREF returns an error code instead of success
> (nothing more to do, already allocated).
> 
> Example of problem (more context can be found in the bug report URL):

Maybe add bit more context in the changelog. Also explain how the
problem can be reproduced.

> Mainline kernel:
> pci 0000:06:01.0: BAR 14: assigned [mem 0x90100000-0xa00fffff] = 256M
> pci 0000:06:04.0: BAR 14: assigned [mem 0xa0200000-0xb01fffff] = 256M
> 
> Patched kernel:
> pci 0000:06:01.0: BAR 14: assigned [mem 0x90100000-0x980fffff] = 128M
> pci 0000:06:04.0: BAR 14: assigned [mem 0x98200000-0xa01fffff] = 128M
> 
> This was using pci=realloc,hpmemsize=128M,nocrs - on the same machine
> with the same configuration, with a Ubuntu mainline kernel and a kernel
> patched with this patch series.
> 
> This patch is vital for the next patch in the series. The next patch

There is no next patch in the patch series ;-)

> allows the user to specify MMIO and MMIO_PREF independently. If the
> MMIO_PREF is set to be very large, this bug will end up more than
> doubling the MMIO size. The bug results in the MMIO_PREF being added to
> the MMIO window, which means doubling if MMIO_PREF size == MMIO size.
> With a large MMIO_PREF, without this patch, the MMIO window will likely
> fail to be assigned altogether due to lack of 32-bit address space.
> 
> Patch notes
> ==========================================================================

Here also the above two lines are not needed.

> Change find_free_bus_resource() to not skip assigned resources with
> non-null parent.
> 
> Add checks in pbus_size_io() and pbus_size_mem() to return success if
> resource returned from find_free_bus_resource() is already allocated.
> 
> This avoids pbus_size_io() and pbus_size_mem() returning error code to
> __pci_bus_size_bridges() when a resource has been successfully assigned
> in a previous pass. This fixes the existing behaviour where space for a
> resource could be reserved multiple times in different parent bridge
> windows. This also greatly reduces the number of failed BAR messages in
> dmesg when Linux assigns resources.
> 
> See related from Logan Gunthorpe (same problem, different solution):
> https://lore.kernel.org/lkml/20190531171216.20532-2-logang@deltatee.com/T/#u

Link: https://lore.kernel.org/lkml/20190531171216.20532-2-logang@deltatee.com/T/#u

> Solves bug report: https://bugzilla.kernel.org/show_bug.cgi?id=203243

Link: https://bugzilla.kernel.org/show_bug.cgi?id=203243

The patch itself looks good to me.