Re: [PATCH v5 1/4] KVM: selftests: Add KVM selftests header files for LoongArch

[maobibo]

On 2023/12/13 下午3:15, zhaotianrui wrote:
> 在 2023/12/13 上午1:18, Sean Christopherson 写道:
> > On Tue, Dec 12, 2023, zhaotianrui wrote:
> > > Hi, Sean:
> > >
> > > I want to change the definition of  DEFAULT_GUEST_TEST_MEM in the common
> > > file "memstress.h", like this:
> > >
> > >   /* Default guest test virtual memory offset */
> > > +#ifndef DEFAULT_GUEST_TEST_MEM
> > >   #define DEFAULT_GUEST_TEST_MEM        0xc0000000
> > > +#endif
> > >
> > > As this address should be re-defined in LoongArch headers.
> >
> > Why?  E.g. is 0xc0000000 unconditionally reserved, not guaranteed to 
> > be valid,
> > something else?
> >
> > > So, do you have any suggesstion?
> >
> > Hmm, I think ideally kvm_util_base.h would define a range of memory 
> > that can be
> > used by tests for arbitrary data.  Multiple tests use 0xc0000000, 
> > which is not
> > entirely arbitrary, i.e. it doesn't _need_ to be 0xc0000000, but 
> > 0xc0000000 is
> > convenient because it's 32-bit addressable and doesn't overlap 
> > reserved areas in
> > other architectures.
In general text entry address of user application on x86/arm64 Linux
is 0x200000, however on LoongArch system text entry address is strange, 
its value 0x120000000.

When DEFAULT_GUEST_TEST_MEM is defined as 0xc0000000, there is 
limitation for guest memory size, it cannot exceed 0x120000000 - 
0xc000000 = 1.5G bytes, else there will be conflict. However
there is no such issue on x86/arm64, since 0xc0000000 is above text 
entry address 0x200000.

The LoongArch link scripts actually is strange, it brings out some 
compatible issues such dpdk/kvm selftest when user applications
want fixed virtual address space.

So here DEFAULT_GUEST_TEST_MEM is defined as 0x130000000 separately, 
maybe 0x140000000 is better since it is 1G super-page aligned for 4K 
page size.

Regards
Bibo Mao

> >
> Thanks for your explanation, and LoongArch want to define 
> DEFAULT_GUEST_TEST_MEM to 0x130000000. As default base address for 
> application loading is 0x120000000, DEFAULT_GUEST_TEST_MEM should be 
> larger than app loading address, so that PER_VCPU_MEM_SIZE can be large 
> enough, and kvm selftests app size is smaller than 256M in generic.
>
> Thanks
> Tianrui Zhao