Re: [PATCH kvm-unit-tests 5/8] arm/arm64: mmu: Remove memory layout assumptions

[Andrew Jones <drjones@redhat.com>]

On Thu, Apr 15, 2021 at 04:48:41PM +0100, Alexandru Elisei wrote:
> Hi Drew,
> 
> On 4/7/21 7:59 PM, Andrew Jones wrote:
> > Rather than making too many assumptions about the memory layout
> > in mmu code, just set up the page tables per the memory regions
> > (which means putting all the memory layout assumptions in setup).
> > To ensure we get the right default flags set we need to split the
> > primary region into two regions for code and data.
> >
> > We still only expect the primary regions to be present, but the
> > next patch will remove that assumption too.
> 
> Nitpick, but we still make assumptions about the memory layout:
> 
> - In setup_mmu(), we limit the maximum linear address to 3GiB, but on arm64 we can
> have memory starting well above that.

True. I need to try and improve that (at least the comment in setup_mmu).
For now, I may just call out that we still assume 3G-4G is available for
our vmalloc region.

> 
> - In mem_init(), we still have the predefined I/O regions.

The commit message points this out. Also, the commit summary specifies
'mmu' for the component from which we're removing the assumptions.

> 
> I don't know if this is a rebasing error or intentional. If it's intentional, I
> think it should be mentioned in the commit message, if only to say they will be
> removed in a later patch (like you do with the primary region).

We never remove all assumptions from mem setup in setup.c. We just make it
easier to bypass.

> 
> >
> > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > ---
> >  lib/arm/asm/setup.h |  1 +
> >  lib/arm/mmu.c       | 26 +++++++++++++++-----------
> >  lib/arm/setup.c     | 22 ++++++++++++++--------
> >  3 files changed, 30 insertions(+), 19 deletions(-)
> >
> > diff --git a/lib/arm/asm/setup.h b/lib/arm/asm/setup.h
> > index c8afb2493f8d..210c14f818fb 100644
> > --- a/lib/arm/asm/setup.h
> > +++ b/lib/arm/asm/setup.h
> > @@ -15,6 +15,7 @@ extern int nr_cpus;
> >  
> >  #define MR_F_PRIMARY		(1U << 0)
> >  #define MR_F_IO			(1U << 1)
> > +#define MR_F_CODE		(1U << 2)
> >  #define MR_F_UNKNOWN		(1U << 31)
> >  
> >  struct mem_region {
> > diff --git a/lib/arm/mmu.c b/lib/arm/mmu.c
> > index a7b7ae51afe3..edd2b9da809b 100644
> > --- a/lib/arm/mmu.c
> > +++ b/lib/arm/mmu.c
> > @@ -20,8 +20,6 @@
> >  
> >  #include <linux/compiler.h>
> >  
> > -extern unsigned long etext;
> > -
> >  #define MMU_MAX_PERSISTENT_MAPS 64
> >  
> >  struct mmu_persistent_map {
> > @@ -208,7 +206,7 @@ void mmu_set_range_sect(pgd_t *pgtable, uintptr_t virt_offset,
> >  
> >  void *setup_mmu(phys_addr_t phys_end)
> >  {
> > -	uintptr_t code_end = (uintptr_t)&etext;
> > +	struct mem_region *r;
> >  
> >  	/* 0G-1G = I/O, 1G-3G = identity, 3G-4G = vmalloc */
> >  	if (phys_end > (3ul << 30))
> > @@ -223,14 +221,20 @@ void *setup_mmu(phys_addr_t phys_end)
> >  
> >  	mmu_idmap = alloc_page();
> >  
> > -	/* armv8 requires code shared between EL1 and EL0 to be read-only */
> > -	mmu_set_range_ptes(mmu_idmap, PHYS_OFFSET,
> > -		PHYS_OFFSET, code_end,
> > -		__pgprot(PTE_WBWA | PTE_RDONLY | PTE_USER));
> > -
> > -	mmu_set_range_ptes(mmu_idmap, code_end,
> > -		code_end, phys_end,
> > -		__pgprot(PTE_WBWA | PTE_USER));
> > +	for (r = mem_regions; r->end; ++r) {
> > +		if (r->flags & MR_F_IO) {
> > +			continue;
> > +		} else if (r->flags & MR_F_CODE) {
> > +			assert_msg(r->flags & MR_F_PRIMARY, "Unexpected code region");
> > +			/* armv8 requires code shared between EL1 and EL0 to be read-only */
> > +			mmu_set_range_ptes(mmu_idmap, r->start, r->start, r->end,
> > +					   __pgprot(PTE_WBWA | PTE_USER | PTE_RDONLY));
> > +		} else {
> > +			assert_msg(r->flags & MR_F_PRIMARY, "Unexpected data region");
> > +			mmu_set_range_ptes(mmu_idmap, r->start, r->start, r->end,
> > +					   __pgprot(PTE_WBWA | PTE_USER));
> > +		}
> > +	}
> 
> This looks good.
> 
> >  
> >  	mmu_set_persistent_maps(mmu_idmap);
> >  
> > diff --git a/lib/arm/setup.c b/lib/arm/setup.c
> > index 9c16f6004e9f..9da5d24b0be9 100644
> > --- a/lib/arm/setup.c
> > +++ b/lib/arm/setup.c
> > @@ -31,6 +31,7 @@
> >  #define NR_INITIAL_MEM_REGIONS 16
> >  
> >  extern unsigned long stacktop;
> > +extern unsigned long etext;
> >  
> >  struct timer_state __timer_state;
> >  
> > @@ -88,10 +89,12 @@ unsigned int mem_region_get_flags(phys_addr_t paddr)
> >  
> >  static void mem_init(phys_addr_t freemem_start)
> >  {
> > +	phys_addr_t code_end = (phys_addr_t)(unsigned long)&etext;
> >  	struct dt_pbus_reg regs[NR_INITIAL_MEM_REGIONS];
> > -	struct mem_region primary, mem = {
> > +	struct mem_region mem = {
> >  		.start = (phys_addr_t)-1,
> >  	};
> > +	struct mem_region *primary = NULL;
> >  	phys_addr_t base, top;
> >  	int nr_regs, nr_io = 0, i;
> >  
> > @@ -110,8 +113,6 @@ static void mem_init(phys_addr_t freemem_start)
> >  	nr_regs = dt_get_memory_params(regs, NR_INITIAL_MEM_REGIONS - nr_io);
> >  	assert(nr_regs > 0);
> >  
> > -	primary = (struct mem_region){ 0 };
> > -
> >  	for (i = 0; i < nr_regs; ++i) {
> >  		struct mem_region *r = &mem_regions[nr_io + i];
> >  
> > @@ -123,7 +124,7 @@ static void mem_init(phys_addr_t freemem_start)
> >  		 */
> >  		if (freemem_start >= r->start && freemem_start < r->end) {
> >  			r->flags |= MR_F_PRIMARY;
> 
> Here we mark mem_regions[nr_io + i] as primary...
> 
> > -			primary = *r;
> > +			primary = r;
> >  		}
> >  
> >  		/*
> > @@ -135,13 +136,18 @@ static void mem_init(phys_addr_t freemem_start)
> >  		if (r->end > mem.end)
> >  			mem.end = r->end;
> >  	}
> > -	assert(primary.end != 0);
> > +	assert(primary);
> >  	assert(!(mem.start & ~PHYS_MASK) && !((mem.end - 1) & ~PHYS_MASK));
> >  
> > -	__phys_offset = primary.start;	/* PHYS_OFFSET */
> > -	__phys_end = primary.end;	/* PHYS_END */
> > +	__phys_offset = primary->start;	/* PHYS_OFFSET */
> > +	__phys_end = primary->end;	/* PHYS_END */
> > +
> > +	/* Split the primary region into two regions; code and data */
> > +	mem.start = code_end, mem.end = primary->end, mem.flags = MR_F_PRIMARY;
> 
> Here we mark mem as primary...

Right, mem is now 

 {
  .start = code_end,
  .end = primary->end,
  .flags = MR_F_PRIMARY
 }

> 
> > +	mem_regions[nr_io + i] = mem;
> 
> And then we set mem_regions[nr_io + nr_regs] to mem, which I think means we can
> end up with two primary memory regions. Am I missing something?
> 
> > +	primary->end = code_end, primary->flags |= MR_F_CODE;

And now primary is

 {
  .start = <the original primary start>,
  .end = code_end,
  .flags = MR_F_PRIMARY|MR_F_CODE,
 }

So there are two primary regions, one for data, one for code. Note, that
we know code_end is within the boundaries of the old full primary region.
All we did was split the region into two.

> 
> Please consider splitting the assignments each on its own line, because it makes
> the code so hard to read (and I assume really easy to miss if we ever change
> something).

Sure

Thanks,
drew