Re: [PATCH] LoongArch: Change the value of macro TRY_EMPTY_VM_SPACE from 0x8000000000 to 0x1000000000.

[Lulu Cheng <chenglulu@loongson.cn>]

在 2023/2/22 下午5:35, WANG Xuerui 写道:
> On 2023/2/22 17:30, Lulu Cheng wrote:
>>
>> 在 2023/2/21 下午9:56, WANG Xuerui 写道:
>>> Hi,
>>>
>>> On 2023/2/21 21:03, Lulu Cheng wrote:
>>>>
>>>> 在 2023/2/21 下午3:41, Xi Ruoyao 写道:
>>>>> On Tue, 2023-02-21 at 15:20 +0800, Lulu Cheng wrote:
>>>>>> Like la264 only has 40 effective bits of virtual address space.
>>>>> I'm OK with the change.  But the VA length is configurable 
>>>>> building the
>>>>> kernel.  Is there any specific reason LA264 has to use the 40-bit
>>>>> configuration, or should we reword the commit message like "for
>>>>> supporting the configuration with less page table level or smaller 
>>>>> page
>>>>> size"?
>>>>
>>>> I consulted with my colleagues who are working on the kernel,
>>>>
>>>> it looks like this:
>>>>
>>>> The la264 chip desgn is physically 40-bit virtual address.
>>>>
>>>> User mode and kernel mode each account for half:
>>>>
>>>> User mode :    0x0-0x7f ffff ffff
>>>>
>>>> Kernel mode:  0xffff ff80 0000 0000 -0xffff ffff ffff ffff
>>>>
>>>> The high bit is the sign extension of bit39.
>>>>
>>> Looking at the comments around the TRY_EMPTY_VM_SPACE definitions, 
>>> they all indicate that the guessed range should be "likely free" -- 
>>> that implies "usable". Given the common VM allocation behavior, we 
>>> want TRY_EMPTY_VM_SPACE to point at a reasonably high place in the 
>>> VA so it's "likely free".
>>>
>>> So IMO the point is, will there be any LoongArch HW in the 
>>> foreseeable future, with less than maybe 40 bits of VA? If the 
>>> answer is "no" then a TRY_EMPTY_VM_SPACE near the 40-bit VA ceiling 
>>> would be appropriate. Otherwise you may have to choose a value near 
>>> or even below a 32-bit VA's upper limit: according to the ISA manual 
>>> Volume 1, Section 2.1.5, "typical VALEN is in the range of [40, 
>>> 48]"; also see Section 5.2.3, RDVA can be as large as 8, so the 
>>> actual VA space could theoretically be as narrow as 40-8=32 bits.
>>
>>
>> Yes, I agree with your point of view this is looking for a "likely 
>> free" virtual memory space.
>>
>>
>> But if I want to support chips with less than 40-bit virtual 
>> addresses, then the value of this macro needs to be set small.
>>
>> I think setting this value small will increase the probability of 
>> virtual address mapping failure.
>
> Not exactly; in case the TYPE_EMPTY_VM_SPACE address happen to be 
> occupied, the mmap will still return something else that's nonzero 
> (consult mmap's man page for details), and will not just blow the 
> process up straight away.
>
> But...
>
>>
>> Chips with less than 40-bit virtual address space are small chips for 
>> embedded use.
>>
>> The purpose of pch is to make compilation faster, but I think we 
>> rarely compile on embedded systems. So this situation may not be 
>> within our consideration.
>>
> Everything makes more sense with this context. Now put these 
> justification into the commit message (after a little bit of rewording 
> maybe) and I think we're good to go then ;-)
OK! Thanks!:-)