linux-kernel - Re: [PATCH v5 10/11] powerpc/pseries/iommu: Make use of DDW for indirect mapping

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Date:   Wed, 21 Jul 2021 17:04:34 +0200
From:   Frederic Barrat <fbarrat@...ux.ibm.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>,
        Leonardo Bras <leobras.c@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        David Gibson <david@...son.dropbear.id.au>,
        kernel test robot <lkp@...el.com>,
        Nicolin Chen <nicoleotsuka@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 10/11] powerpc/pseries/iommu: Make use of DDW for
 indirect mapping



On 21/07/2021 05:32, Alexey Kardashevskiy wrote:
>>> +        struct iommu_table *newtbl;
>>> +        int i;
>>> +
>>> +        for (i = 0; i < ARRAY_SIZE(pci->phb->mem_resources); i++) {
>>> +            const unsigned long mask = IORESOURCE_MEM_64 | 
>>> IORESOURCE_MEM;
>>> +
>>> +            /* Look for MMIO32 */
>>> +            if ((pci->phb->mem_resources[i].flags & mask) == 
>>> IORESOURCE_MEM)
>>> +                break;
>>> +        }
>>> +
>>> +        if (i == ARRAY_SIZE(pci->phb->mem_resources))
>>> +            goto out_del_list;
>>
>>
>> So we exit and do nothing if there's no MMIO32 bar?
>> Isn't the intent just to figure out the MMIO32 area to reserve it when 
>> init'ing the table? In which case we could default to 0,0
>>
>> I'm actually not clear why we are reserving this area on pseries.
> 
> 
> 
> If we do not reserve it, then the iommu code will allocate DMA pages 
> from there and these addresses are MMIO32 from the kernel pov at least. 
> I saw crashes when (I think) a device tried DMAing to the top 2GB of the 
> bus space which happened to be a some other device's BAR.


hmmm... then figuring out the correct range needs more work. We could 
have more than one MMIO32 bar. And they don't have to be adjacent. I 
don't see that we are reserving any range on the initial table though 
(on pseries).

   Fred