Re: CVMSEG Emulation

[Jiaxun Yang]

> 2022年12月9日 17:44，Christopher Wrogg <cwrogg@umich.edu> 写道：
> 
> I tried both.
> 
> Option 1 
>     What I did:
>         #undef TARGET_VIRT_ADDR_SPACE_BITS and #define 
> TARGET_VIRT_ADDR_SPACE_BITS 64
>     The Result:
>         perror reports "Cannot allocate memory"
> Option 2: 
>     What I did:
>         TARGET_VIRT_ADDR_SPACE_BITS for me is 30 so I masked by 0x3FFFFFFF
>     The Result:
>         The segfault persists and gdb reports the memory as inaccessible.

Hmm this looks wired for me, by no chance TARGET_VIRT_ADDR_SPACE_BITS for MIPS
can be 30, on N64 ABI build it should be 48 and 32 for N32 or O32 build.

It is defined in target/mips/cpu-param.h .

Thanks.

> 
> On Thu, Dec 8, 2022 at 4:55 PM Jiaxun Yang <jiaxun.yang@flygoat.com> wrote:
> 
> Hi,
> 
> This address range is located in KSEG3… Doesn’t seems to be a good location
> for userspace program.
> 
> I think you have two options to make target_mmap work, the first would be 
> rising
> TARGET_VIRT_ADDR_SPACE_BITS to 64 bit. That may break some user space
> applications storing pointer tags on higher bits.
> 
> The second would be mask CVMSEG base with TARGET_VIRT_ADDR_SPACE_BITS
> before mmap, As higher VM address bits will be dropped when addressing guest 
> VM,
> that should provide a similar behaviour. Though you’ll have multiple alias 
> for CVMSEG in
> memory and application will be able to access CVMSEG with bits higher than
> TARGET_VIRT_ADDR_SPACE_BITS set to any value. Don’t know if it will break 
> anything,
> AFAIK normal applications won't use this range.
> 
> Thanks
> - Jiaxun 
> 
> 
> > 2022年12月8日 15:08，Christopher Wrogg <cwrogg@umich.edu> 写道：
> > 
> > In userspace emulation how do I make a set of addresses always valid and 
> > initialized to 0 even though the process does not map it in? In particular 
> > I want to map the CVMSEG for Cavium qemu-mips64 and qemu-mipsn32. The 
> > addresses would be 0xFFFFFFFFFFFF8000 - 0xFFFFFFFFFFFFBFFF. I've looked at 
> > target_mmap but it can't handle addresses that large. The lack of an 
> > emulated mmu for 64 bit guests is going to be a problem.
>