Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report_event helper
From: Jason Gunthorpe <jgg@nvidia.com>
Date: 2025-01-13 19:21:51
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On Sun, Jan 12, 2025 at 09:37:41PM -0800, Nicolin Chen wrote:
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I supposed we will need a way to indicate lost events to userspace on top of this?Perhaps another u32 flag in the arm_smmuv3_vevent struct to report an overflow. That said, what userspace/VMM will need to do with it?Trigger the above code in the VM somehow?I found two ways of forwarding an overflow flag: 1. Return -EOVERFLOW to read(). But it cannot return the read bytes any more:
You could not return any bytes, it would have to be 0 bytes read, ie immediately return EOVERFLOW and do nothing else. Returning EOVERFLOW from read would have to also clear the overflow indicator. The other approach would be to add a sequence number to each event and let userspace detect the non-montonicity. It would require adding a header to the native ARM evt.
quoted hunk ↗ jump to hunk
2. Return EPOLLERR via pollfd.revents. But it cannot use POLLERR for other errors any more: --------------------------------------------------@@ -420,2 +421,4 @@ static __poll_t iommufd_eventq_fops_poll(struct file *filep, poll_wait(filep, &eventq->wait_queue, wait); + if (test_bit(IOMMUFD_VEVENTQ_ERROR_OVERFLOW, veventq->errors)) + return EPOLLERR; mutex_lock(&eventq->mutex);
But then how do you clear the error? I've only seen POLLERR used for fatal conditions so there is no recovery, it is permanent. Jason