PCI device memory from BAR under qemu alwys zero

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MigthyMax

Re: PCI device memory from BAR under qemu alwys zero

Posted on: 2023/9/15 10:12 #21

Just popping in

@Hans

Quote:

If it's the latter, then check whether the master and memory enable bits are set in the PCI_COMMAND register (enable I/O accesses
too if you have an I/O BAR). You need to enable those bits in order to access the memory.

I think that could be what I'm missing. At least this I never read/heard of yet, compared to what balaton, jeorg etc wrote. That
all I knew from the virtio spec etc.

Another topic which comes to my mind. do I have take care of the cpu caches? Maybe I just writing/reading into/from the cache and the real memory isn't updated?

MigthyMax

Re: PCI device memory from BAR under qemu alwys zero

Posted on: 2023/9/15 12:28 #22

Just popping in

@Hans

Yeap that made the trick:


pciDevice->WriteConfigWord( PCI_COMMAND,0x002 | pciDevice->ReadConfigWord( PCI_COMMAND ) );

Now i get other stuff than zeros!

Hans

Re: PCI device memory from BAR under qemu alwys zero

Posted on: 2023/9/15 14:55 #23

Home away from home

@MigthyMax

Great that you got it working.

Quote:

Another topic which comes to my mind. do I have take care of the cpu caches? Maybe I just writing/reading into/from the cache and the real memory isn't updated?

No. Cache behaviour isn't emulated. You can assume that the caches are always coherent, and flushing/invalidating isn't necessary.

At least, I've never seen physical virtio hardware (which could theoretically be done).

Hans

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