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Re: [Qemu-devel] about correctness of IDE emulation

From: Huaicheng Li
Subject: Re: [Qemu-devel] about correctness of IDE emulation
Date: Mon, 14 Mar 2016 22:09:17 -0500

> On Mar 13, 2016, at 8:42 PM, Fam Zheng <address@hidden> wrote:
> On Sun, 03/13 14:37, Huaicheng Li (coperd) wrote:
>> Hi all, 
>> What I’m confused about is that:
>> If one I/O is too large and may need several rounds (say 2) of DMA transfers,
>> it seems the second round transfer begins only after the completion of the
>> first part, by reading data from **IDEState**. But the IDEState info may have
>> been changed by VCPU threads (by writing new I/Os to it) when the first
>> transfer finishes. From the code, I see that IDE r/w call back function will
>> continue the second transfer by referencing IDEState’s information. Wouldn’t
>> this be problematic? Am I missing anything here?
> Can you give an concrete example? I/O in VCPU threads that changes IDEState
> must also take care of the DMA transfers, for example ide_reset() has
> blk_aio_cancel and clears s->nsectors. If an I/O handler fails to do so, it is
> a bug.
> Fam

I get it now. ide_exec_cmd() can only proceed when BUSY_STAT|DRQ_STAT is not 
When the 2nd DMA transfer continues, BUSY_STAT | DRQ_STAT is already
set, i.e., no other new ide_exec_cmd() can enter. BSUY or DRQ is removed only 
all DMA transfers are done, after which new writes to IDE are allowed. Thus 
it’s safe.

Thanks, Fam & Stefan.

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