RE: [PATCH v4 4/4] RISC-V: Add arch functions to support hibernation/suspend-to-disk

[JeeHeng Sia <jeeheng.sia@starfivetech.com>]

> -----Original Message-----
> From: Andrew Jones <ajones@ventanamicro.com>
> Sent: Tuesday, 28 February, 2023 3:30 PM
> To: JeeHeng Sia <jeeheng.sia@starfivetech.com>
> Cc: paul.walmsley@sifive.com; palmer@dabbelt.com; aou@eecs.berkeley.edu; linux-riscv@lists.infradead.org; linux-
> kernel@vger.kernel.org; Leyfoon Tan <leyfoon.tan@starfivetech.com>; Mason Huo <mason.huo@starfivetech.com>
> Subject: Re: [PATCH v4 4/4] RISC-V: Add arch functions to support hibernation/suspend-to-disk
> 
> On Tue, Feb 28, 2023 at 06:33:59AM +0000, JeeHeng Sia wrote:
> >
> >
> > > -----Original Message-----
> > > From: Andrew Jones <ajones@ventanamicro.com>
> > > Sent: Tuesday, 28 February, 2023 1:05 PM
> > > To: JeeHeng Sia <jeeheng.sia@starfivetech.com>
> > > Cc: paul.walmsley@sifive.com; palmer@dabbelt.com; aou@eecs.berkeley.edu; linux-riscv@lists.infradead.org; linux-
> > > kernel@vger.kernel.org; Leyfoon Tan <leyfoon.tan@starfivetech.com>; Mason Huo <mason.huo@starfivetech.com>
> > > Subject: Re: [PATCH v4 4/4] RISC-V: Add arch functions to support hibernation/suspend-to-disk
> > >
> > > On Tue, Feb 28, 2023 at 01:32:53AM +0000, JeeHeng Sia wrote:
> > > > > > > > 	load image;
> > > > > > > > loop:	Create pbe chain, return error if failed;
> > > > > > >
> > > > > > > This loop pseudocode is incomplete. It's
> > > > > > >
> > > > > > > loop:
> > > > > > >         if (swsusp_page_is_forbidden(page) && swsusp_page_is_free(page))
> > > > > > > 	   return page_address(page);
> > > > > > > 	Create pbe chain, return error if failed;
> > > > > > > 	...
> > > > > > >
> > > > > > > which I pointed out explicitly in my last reply. Also, as I asked in my
> > > > > > > last reply (and have been asking four times now, albeit less explicitly
> > > > > > > the first two times), how do we know at least one PBE will be linked?
> > > > > > 1 PBE correspond to 1 page, you shouldn't expect only 1 page is saved.
> > > > >
> > > > > I know PBEs correspond to pages. *Why* should I not expect only one page
> > > > > is saved? Or, more importantly, why should I expect more than zero pages
> > > > > are saved?
> > > > >
> > > > > Convincing answers might be because we *always* put the restore code in
> > > > > pages which get added to the PBE list or that the original page tables
> > > > > *always* get put in pages which get added to the PBE list. It's not very
> > > > > convincing to simply *assume* that at least one random page will always
> > > > > meet the PBE list criteria.
> > > > >
> > > > > > Hibernation core will do the calculation. If the PBEs (restore_pblist) linked successfully, the hibernated image will be restore
> else
> > > > > normal boot will take place.
> > > > > > > Or, even more specifically this time, where is the proof that for each
> > > > > > > hibernation resume, there exists some page such that
> > > > > > > !swsusp_page_is_forbidden(page) or !swsusp_page_is_free(page) is true?
> > > > > > forbidden_pages and free_pages are not contributed to the restore_pblist (as you already aware from the code). Infact, the
> > > > > forbidden_pages and free_pages are not save into the disk.
> > > > >
> > > > > Exactly, so those pages are *not* going to contribute to the greater than
> > > > > zero pages. What I've been asking for, from the beginning, is to know
> > > > > which page(s) are known to *always* contribute to the list. Or, IOW, how
> > > > > do you know the PBE list isn't empty, a.k.a restore_pblist isn't NULL?
> > > > Well, this is keep going around in a circle, thought the answer is in the hibernation code. restore_pblist get the pointer from the
> PBE,
> > > and the PBE already checked for validity.
> > >
> > > It keeps going around in circles because you keep avoiding my question by
> > > pointing out trivial linked list code. I'm not worried about the linked
> > > list code being correct. My concern is that you're using a linked list
> > > with an assumption that it is not empty. My question has been all along,
> > > how do you know it's not empty?
> > >
> > > I'll change the way I ask this time. Please take a look at your PBE list
> > > and let me know if there are PBEs on it that must be there on each
> > > hibernation resume, e.g. the resume code page is there or whatever.
> > Just to add on, it is not "my" PBE list but the list is from the hibernation core. As already draw out the scenarios for you, checking
> should be done at the initialization phase.
> 
> Your PBE list is your instance of the PBE list when you resume your
> hibernation test. I'm simply asking you to dump the PBE list while
> you resume a hibernation, and then tell me what's there.
> 
> Please stop thinking about the trivial details of the code, like which
> file a variable is in, and start thinking about how the code is being
> used. A PBE list is a concept, your PBE list is an instance of that
> concept, the code, which is the least interesting part, is just an
> implementation of that concept. First, I want to understand the concept,
> then we can worry about the code.
> 
Dear Andrew, perhaps a conference call is better? otherwise it is going to waste the time in typing...Let me know how to join the call with you....thank you.
> drew
> 
> > >
> > > > Can I suggest you to submit a patch to the hibernation core?
> > >
> > > Why? What's wrong with it?
> > >
> > > Thanks,
> > > drew