Aneesh Kumar K.V <[email protected]> writes:
> Will Deacon <[email protected]> writes:
>
>> On Wed, Oct 07, 2026 at 11:04:05AM +0100, Catalin Marinas wrote:
>>> On Tue, Oct 06, 2026 at 10:49:17PM +0100, Will Deacon wrote:
>>> > On Thu, Sep 24, 2026 at 11:37:54AM +0530, Aneesh Kumar K.V (Arm) wrote:
>>> > > @@ -496,7 +516,8 @@ swiotlb_select_pool_policy(unsigned int flags)
>>> > > if (swiotlb_force_disable)
>>> > > return SWIOTLB_POOL_NONE;
>>> > >
>>> > > - if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
>>> > > + if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&
>>> > > + !restricted_dma_pool_present)
>>> > > return SWIOTLB_POOL_CC_GUEST;
>>> >
>>> > I think this check on the restricted DMA pool is too general -- the pool
>>> > could be tied to a specific DMA-capable peripheral and so treating its
>>> > presence as a global property isn't right.
>>>
>>> I agree it's a hack but that was the simplest way to avoid the pVMs
>>> getting a bounce buffer after this patch. More than happy to leave it
>>> out and reduce the buffer on cmdline or we come up with some better
>>> heuristics.
>>
>> Hrm, that does mean that reverting just this part will regress pVMs
>> because they'll suddenly be allocating a tonne more memory for an
>> entirely unused swiotlb buffer. So I think I'd prefer to drop the entire
>> series until this has been worked out properly.
>>
>>> Another option could be the arch code passing another flag that it
>>> doesn't want an encrypted pool (e.g. when running in a pKVM guest) but I
>>> don't particularly this either. The arch code doesn't know whether
>>> there's an alternative pool.
>>
>> At that point, the default size may as well be driven by the
>> drivers/virt/coco driver.
>>
>>> That said, such heuristics should have been a separate patch to make it
>>> easier to review/drop.
>>
>> I think the only right way to get a semi-accurate heuristic is to take
>> into account the set of dma-capable devices that will use the swiotlb
>> pool, but that's fiddly and should probably be tackled as a separate
>> series. Maybe a simpler hack in that direction would be to take the
>> SWIOTLB_POOL_CC_GUEST if _any_ device is going to use swiotlb? You'll
>> run into the usual problem of not being able to tell if a device is
>> DMA-capable or not, but you could probably look for a global restricted
>> DMA pool and, if that doesn't exist, check for per-device restricted pools
>> on dma-coherent devices (since restricted DMA isn't supported by ACPI) as
>> a reasonable approximation.
>
> So, something like this?
>
> if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&
> swiotlb_cc_guest_needs_default_pool())
> return SWIOTLB_POOL_CC_GUEST;
>
Detecting a DMA-capable device is not straightforward, and if we get it
wrong, we will enable SWIOTLB_POOL_CC_GUEST unnecessarily. Would the
code below be a reasonable approximation of what you suggested?
Another option would be to make swiotlb_cc_guest_needs_default_pool() a
weak function that architectures can override. arm64 pKVM could then use
a different scheme (for this patch series default to false). Would that
be preferable?
#ifdef CONFIG_DMA_RESTRICTED_POOL
static bool __init swiotlb_of_dma_candidate(struct device_node *np)
{
struct device_node *node __free(device_node) = of_node_get(np);
/*
* Ignore nodes that don't have compatible and reg property
* So we don't wrongly consider a node as device node.
*/
if (!of_property_present(np, "compatible") ||
!of_property_present(np, "reg"))
return false;
while (node) {
if (!of_device_is_available(node))
return false;
/*
* Ignore reserved-memory nodes because that have compatible
* and reg property
*/
if (node->parent == of_root &&
of_node_name_eq(node, "reserved-memory"))
return false;
node = of_get_next_parent(node);
}
return of_dma_is_coherent(np);
}
static bool __init swiotlb_of_dma_needs_default_pool(struct device_node *np)
{
if (!of_dma_get_restricted_pool(np))
return true;
return false;
}
#endif
static bool __init swiotlb_cc_guest_needs_default_pool(void)
{
#ifdef CONFIG_DMA_RESTRICTED_POOL
struct device_node *np;
bool found = false;
if (!of_root)
return true;
/*
* DMA capability is not explicitly described for every DT device.
* Use coherent, addressed device nodes as an approximation, and keep
* guest sizing unless every candidate has an initialized restricted
* pool.
*/
for_each_of_allnodes(np) {
if (!swiotlb_of_dma_candidate(np))
continue;
found = true;
if (swiotlb_of_dma_needs_default_pool(np))
return true;
}
return !found;
#else
return true;
#endif
}
