Re: [POC][RFC][PATCH 0/2] pstore/mm/x86: Add wildcard memmap to map pstore consistently

[Mike Rapoport]

On Wed, May 01, 2024 at 12:09:04PM -0400, Steven Rostedt wrote:
> On Wed, 1 May 2024 18:30:40 +0300
> Mike Rapoport <rppt@xxxxxxxxxx> 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.

On x86 memblock allocates from top of the memory. As this will run later
than e820, the allocation will be lower than from e820, but still close to
the top of the memory.
 
> I would still like to get the highest address possible.
> 
> -- Steve

-- 
Sincerely yours,
Mike.