[virtio-comment] Re: [PATCH V2 6/6] virtio-pci: implement dirty page tracking

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 03, 2023 at 06:34:37PM +0800, Zhu Lingshan wrote:
> +\item[\field{bitmap_addr}]
> +	The driver use this to set the address of the bitmap which records the dirty pages
> +	caused by the device.
> +	Each bit in the bitmap represents one memory page, bit 0 in the bitmap
> +	reprsents page 0 at address 0, bit 1 represents page 1, and so on in a linear manner.
> +	When \field{enable} is set to 1 and the device writes to a memory page,
> +	the device MUST set the corresponding bit to 1 which indicating the page is dirty.
> +\item[\field{bitmap_length}]
> +	The driver use this to set the length in bytes of the bitmap.
> +\end{description}
> +
> +\devicenormative{\subsubsection}{Memory Dirty Pages Tracker Capability}{Virtio Transport Options / Virtio Over PCI Bus / Memory Dirty Pages Tracker Capability}
> +
> +The device MUST NOT set any bits beyond bitmap_length when reporting dirty pages.
> +
> +To prevent a read-modify-write procedure, if a memory page is dirty,
> +optionally the device is permitted to set the entire byte, which encompasses the relevant bit, to 1.
> +
> +The device MAY increase \field{gra_power} to reduce \field{bitmap_length}.
> +
> +The device must ignore any writes to \field{pasid} if PASID Extended Capability is absent or
> +the PASID functionality is disabled in PASID Extended Capability


I have to say this is going to work very badly when the number of dirty
pages is small: you will end up scanning and re-scanning all of bitmap.
And the resolution is apparently 8 pages? You have just multiplied
the migration bandwidth by a factor of 8.

-- 
MST


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