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

[Lulu Cheng <chenglulu@loongson.cn>]

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.

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.