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Date: Wed, 1 May 2024 12:09:04 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Mike Rapoport <rppt@kernel.org>
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Subject: Re: [POC][RFC][PATCH 0/2] pstore/mm/x86: Add wildcard memmap to map
 pstore consistently
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On Wed, 1 May 2024 18:30:40 +0300
Mike Rapoport <rppt@kernel.org> wrote:

> > > /*
> > >  * Parse early_reserve_mem=nn:align:name
> > >  */
> > > static int __init early_reserve_mem(char *p)
> > > {
> > > 	phys_addr_t start, size, align;
> > > 	char *oldp;
> > > 	int err;
> > > 
> > > 	if (!p)
> > > 		return -EINVAL;
> > > 
> > > 	oldp = p;
> > > 	size = memparse(p, &p);
> > > 	if (p == oldp)
> > > 		return -EINVAL;
> > > 
> > > 	if (*p != ':')
> > > 		return -EINVAL;
> > > 
> > > 	align = memparse(p+1, &p);
> > > 	if (*p != ':')
> > > 		return -EINVAL;
> > > 
> > > 	start = memblock_phys_alloc(size, align);  
> > 
> > So this will allocate the same physical location for every boot, if booting
> > the same kernel and having the same physical memory layout?  
> 
> Up to kaslr that might use that location for the kernel image.
> But it's the same as allocating from e820 after kaslr.
> 
> And, TBH, I don't have good ideas how to ensure the same physical location
> with randomization of the physical address of the kernel image.

I'll try it out. Looking at arch/x86/boot/compressed/kaslr.c, if I read the
code correctly, it creates up to a 100 slots to store the kernel.

The method I used was to make sure that the allocation was always done at
the top address of memory, which I think would in most cases never be
assigned by KASLR.

This looks to just grab the next available physical address, which KASLR
can most definitely mess with.

I would still like to get the highest address possible.

-- Steve