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

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Date:   Wed, 13 Dec 2023 15:42:26 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     zhaotianrui <zhaotianrui@...ngson.cn>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Shuah Khan <shuah@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vishal Annapurve <vannapurve@...gle.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev,
        Peter Xu <peterx@...hat.com>,
        Vipin Sharma <vipinsh@...gle.com>, huangpei@...ngson.cn
Subject: Re: [PATCH v5 1/4] KVM: selftests: Add KVM selftests header files for
 LoongArch



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