Re: [PATCH v2 2/2] drivers/char: mark extra reserved device memory in memory map

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Mon, Mar 18, 2024 at 02:48:09PM +0100, Jan Beulich wrote:
> On 12.03.2024 17:25, Marek Marczykowski-Górecki wrote:
> > The IOMMU driver checks if RMRR/IVMD are marked as reserved in memory
> > map. This should be true for addresses coming from the firmware, but
> > when extra pages used by Xen itself are included in the mapping, those
> > are taken from usable RAM used. Mark those pages as reserved too.
> > 
> > Not marking the pages as reserved didn't caused issues before due to
> > another a bug in IOMMU driver code, that was fixed in 83afa3135830
> > ("amd-vi: fix IVMD memory type checks").
> > 
> > Failing to reserve memory will lead to panic in IOMMU setup code. And
> > not including the page in IOMMU mapping will lead to broken console (due
> > to IOMMU faults). The pages chosen by the XHCI console driver should
> > still be usable by the CPU though, and the console code already can deal
> > with too slow console by dropping characters (and console not printing
> > anything is a special case of "slow"). When reserving fails print an error
> > message showing which pages failed and who requested them. This should
> > be enough hint to find why XHCI console doesn't work.
> > 
> > Fixes: 3a1a7b809ffa "drivers/char: mark DMA buffers as reserved for the 
> > XHCI"
> > Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> > Alternative error handling could be a panic, but with this version I
> > think it can be avoided. And not panicing gives a better chance to
> > actually see the error message (from the hopefully started dom0),
> > especially as the affected driver is the console one.
> > 
> > The reserve_e820_ram() is x86-specific. Is there some equivalent API for
> > ARM, or maybe even some abstract one? That said, I have no way to test
> > XHCI console on ARM, I don't know if such hardware even exists...
> 
> These are normal PCI devices, so I don't see why they shouldn't be usable
> on non-x86 systems. But this is all okay as long as XHCI depends on X86
> in Kconfig.

That's why I'm asking for similar API for ARM. The x86-specific part
here is only about IOMMU, other parts should work just fine (but as I
said, I have no way to test).

Anyway, I'll leave it to whoever will need this driver on ARM (or other
arch).

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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