Re: [Qemu-devel] about I/O

[Stefan Hajnoczi]

On Tue, Dec 29, 2015 at 03:36:07PM +0800, 浩樊啊 wrote:
> qemu use paio or libaio finish I/O reqeusts,does that mean that when a sync 
> request comes from a vm becomes async request in the host?

Yes, although storage controllers (virtio-blk, SCSI, SATA, and even IDE
in DMA mode) are asynchronous at the hardware interface level.  That
means the guest OS submits an I/O request to the device and the guest
CPU continues running until the I/O completion interrupt occurs.

The exceptions are old hardware interfaces like some of the SD Card
controllers or maybe floppy where a single hardware register read/write
actually involves synchronous I/O (the guest cannot execute CPU
instructions while the I/O request is happening).

Stefan

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