Re: MIPS cache bypass on custom board

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi Alexander,

On 12/13/19 7:59 PM, Bensch, Alexander wrote:
> Hi all,
> 
> Currently stuck on a problem in QEMU 4.0.0. I’m trying to implement a 
> custom device using a MIPS 24Kc CPU. The device boots from an SPI flash 
> device that is mapped to 0x9F000000 (physical address 0x1F000000). I got 
> the initial load and execute working by direct loading a flash dump to a 
> MemoryRegion based at 0x1F000000, which worked great until the ROM 
> needed to access the SPI registers that are addressed to 0xBF000000 
> (/also /physical address 0x1F000000). QEMU cannot differentiate reads 
> and writes to 0xBF000000 from reads and writes to 0x9F000000.
> 
> Initially I assumed this was a caching problem, as I know that the SPI 
> registers are located in the KSEG1 memory segment which uses uncached 
> writes, while the flash mapping is in KSEG0 with cached writes. I also 
> can see that QEMU has logic to handle caching in a few source files 
> within /targets/mips//. However, when I read from addresses in the KSEG1 
> region, I still see contents from the KSEG0 region.
> 
> My question is whether there is any way to configure a MIPS board such 
> that I can correctly bypass the cache for KSEG1 as expressed by the MIPS 
> documentation?

Unfortunately QEMU doesn't model microarchitecture, thus no cache is 
modelled, meaning KSEG1 is the same as KSEG0.

Peter Maydell had the similar problem you describe last year:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg556999.html

> 
> Apologies if details are lacking. Please request more info if needed.
> 
> Thank you,
> 
> *Alex Bensch*