Re: [PATCH V2 1/2] kexec: arm64: create identity page table to be used in purgatory

[Pratyush Anand <panand@redhat.com>]

Hi James,

Thanks for your review.

On Tuesday 17 January 2017 10:55 PM, James Morse wrote:
> Hi Pratyush,
>
> On 19/12/16 07:13, Pratyush Anand wrote:
>> Purgatory sha verification is very slow when D-cache is not enabled there.
>> We need to enable MMU as well to enable D-Cache.Therefore,we need to an
>> identity mapped page table in purgatory.
>>
>> Since debugging is very difficult in purgatory therefore we prefer to do as
>> much work as possible in kexec.
>>
>> This patch prepares page table for purgatory in advance. We support only 4K
>> page table,because it will be available on most of the platform. This page
>> table is passed to the purgatory as a new segment.
>>
>> VA bit is fixed as 48, page table level is 3 where 3rd level page table
>> contains 2M block entries.
>
>> diff --git a/kexec/arch/arm64/kexec-arm64.c b/kexec/arch/arm64/kexec-arm64.c
>> index 04fd3968bb52..c2c8ff1b6940 100644
>> --- a/kexec/arch/arm64/kexec-arm64.c
>> +++ b/kexec/arch/arm64/kexec-arm64.c
>> @@ -24,6 +24,45 @@
>>  #include "kexec-syscall.h"
>>  #include "arch/options.h"
>>
>> +/*
>> + * kexec creates identity page table to be used in purgatory so that
>> + * dcache verification becomes faster.
>> + *
>> + * These are the definitions to be used by page table creation routine.
>> + *
>> + * Only 4K page table, 3 level, 2M block mapping, 48bit VA is supported
>> + */
>> +#define PGDIR_SHIFT		39
>> +#define PUD_SHIFT		30
>> +#define PMD_SHIFT		21
>> +#define PTRS_PER_PGD		0x1FF
>> +#define PTRS_PER_PUD		0x1FF
>> +#define PTRS_PER_PMD		0x1FF
>
> Aren't these 0x200 for 4K pages in the kernel? It looks like you use them as a
> mask instead.
>

will use definitions as in kernel.

>
>> +#define PMD_TYPE_TABLE		(3UL << 0)
>> +#define PMD_TYPE_SECT		(1UL << 0)
>> +#define PMD_SECT_AF		(1UL << 10)
>> +#define PMD_ATTRINDX(t)		((unsigned long)(t) << 2)
>
>> +#define MT_NORMAL		4
>
> This needs to correspond to the part of MAIR that describes the memory type for
> 'normal'. You define MT_NORMAL again in the next patch, it would be better to
> share the definition so they can't be different. (I don't see why you can't use 0!)

Hummm...Had thought to share the definition, but inclusion of header 
file into purgatory code looked like ugly. But I see 
purgatory/purgatory.c is including "../kexec/kexec-sha256.h". So, 
probably I can do that similarly.

Yes, can use 0 as well, as there is no other memory type now.

>
>
>> +#define PMD_FLAGS_NORMAL	(PMD_TYPE_SECT | PMD_SECT_AF)
>> +#define MMU_FLAGS_NORMAL	(PMD_ATTRINDX(MT_NORMAL) | PMD_FLAGS_NORMAL)
>
> The SH and AP bits are left as zero, which means non-shareable, read/writeable,
> (which I think is fine).
>
>
>> +#define SECTION_SIZE		(2 * 1024 * 1024)
>> +#define PAGE_SIZE		(4 * 1024)
>> +/* Since we are using 3 level of page tables, therefore minimum number of
>> + * table will be 3. Most likely we will never need more than 3. Each entry
>> + * in level 3 page table can map 2MB memory area. Thus a level 3 page table
>> + * indexed by bit 29:21 can map a total of 1G memory area. Therefore, if
>> + * any segment crosses 1G boundary, then we will need one more level 3
>> + * table. Similarly, level 2 page table indexed by bit 38:30 can map a
>> + * total of 512G memory area. If segment addresses are more than 512G apart
>> + * then we will need two more table for each such block. We do not expect
>> + * any memory segment to cross 512G boundary, however if we will ever wish
>> + * to support uart debugging in purgatory then that might cross the
>> + * boundary and therefore additional 2 more table space. Thus we will need
>> + * maximum of 6 table space.
>
> Surely we only need 6 page_size table entries if we support uart debugging,
> which your 'if we ever' suggests we don't. So surely we only need 3, and a
> comment that this needs expanding to 6 if we need to map two distinct areas.
>

OK, will use 3 now and write comments accordingly.

>
>> + */
>> +#define MAX_PGTBLE_SZ	(6 * 4096)
>> +static int next_tbl_cnt = 1;
>> +
>>  /* Global varables the core kexec routines expect. */
>>
>>  unsigned char reuse_initrd;
>> @@ -316,6 +355,117 @@ unsigned long arm64_locate_kernel_segment(struct kexec_info *info)
>>  	return hole;
>>  }
>>
>> +static unsigned long *create_table_entry(unsigned long *pgtbl_buf,
>> +		unsigned long pgtbl_mem, unsigned long *tbl,
>> +		unsigned long virt, int shift,
>> +		unsigned long ptrs)
>> +{
>> +	unsigned long index, desc, offset;
>> +
>> +	index = (virt >> shift) & ptrs;
>> +	/* check if we have allocated a table already for this index */
>> +	if (tbl[index]) {
>> +		/*
>> +		 * table index will have entry as per purgatory page table
>> +		 * memory. Find out corresponding buffer address of table.
>> +		 */
>> +		desc = tbl[index] & ~3UL;
>> +		offset = desc - pgtbl_mem;
>
> wait .. no .. pgtbl_mem is also a guest physical address, so this is fine... its
> not obvious at first glance!  You could do with a typedef to make it clear which
> addresses are the guests (pgtable_mem) and which are the hosts (pgtable_buf).
> Something like phys_addr_t?

so may be host_addr_t and guest_addr_t??

>
>
>> +		return &pgtbl_buf[offset >> 3];
>> +	}
>> +
>> +	/*
>> +	 * Always write page table content as per page table memory allocated
>> +	 * for purgaory area, but return corresponding buffer area alloced
>
> Nit: purgatory, allocated,

ok.

>
>> +	 * in kexec
>> +	 */
>> +	if (next_tbl_cnt > 5)
>> +		die("%s: No more memory for page table\n", __func__);
>
> die()? With a bit of juggling can't we return an error so we never try to enable
> the MMU+dcache instead?
>

OK. can do that.

>
>> +
>> +	tbl[index] = (pgtbl_mem + PAGE_SIZE * next_tbl_cnt) | PMD_TYPE_TABLE;
>> +
>> +	return &pgtbl_buf[(next_tbl_cnt++ * PAGE_SIZE) >> 3];
>> +}
>> +
>> +static void craete_block_entry(unsigned long *tbl, unsigned long flags,
>> +		unsigned long phys, unsigned long virt)
>
> Why have separate phys and virt parameters if all this ever does is idmap?

Agreed.

>
>
>> +{
>> +	unsigned long index;
>> +	unsigned long desc;
>> +
>> +	index = (virt >> PMD_SHIFT) & PTRS_PER_PMD;
>
> Copying the pmd_index() macro would make this clearer.

Right.

>
>
>> +	desc = (phys >> PMD_SHIFT) << PMD_SHIFT;
>
> Looks like you wanted a PMD_MASK.

Right.

>
>
>> +	desc |= flags;
>> +	tbl[index] = desc;
>> +}
>> +
>> +static void create_identity_entry(unsigned long *pgtbl_buf,
>> +		unsigned long pgtbl_mem, unsigned long virt,
>> +		unsigned long flags)
>> +{
>> +	unsigned long *tbl = pgtbl_buf;
>> +
>> +	tbl = create_table_entry(pgtbl_buf, pgtbl_mem, tbl, virt,
>> +			PGDIR_SHIFT, PTRS_PER_PGD);
>> +	tbl = create_table_entry(pgtbl_buf, pgtbl_mem, tbl, virt,
>> +			PUD_SHIFT, PTRS_PER_PUD);
>> +	craete_block_entry(tbl, flags, virt, virt);
>
> Nit: create

Ok.

>
>> +}
>> +
>> +/**
>> + * arm64_create_pgtbl_segment - Create page table segments to be used by
>> + * purgatory. Page table will have entries to access memory area of all
>> + * those segments which becomes part of sha verification in purgatory.
>> + * Additionaly, we also create page table for purgatory segment as well.
>
> Nit: additionally,

Ok.

>
>> + */
>> +
>> +static int arm64_create_pgtbl_segment(struct kexec_info *info,
>> +		unsigned long hole_min, unsigned long hole_max)
>> +{
>> +	unsigned long *pgtbl_buf;
>> +	int i;
>> +	unsigned long mstart, mend, pgtbl_mem;
>> +	unsigned long purgatory_base, purgatory_len;
>> +
>> +	pgtbl_buf = xmalloc(MAX_PGTBLE_SZ);
>> +	memset(pgtbl_buf, 0, MAX_PGTBLE_SZ);
>
>> +	pgtbl_mem = add_buffer_phys_virt(info, pgtbl_buf, MAX_PGTBLE_SZ,
>> +			MAX_PGTBLE_SZ, PAGE_SIZE, hole_min, hole_max, 1, 0);
>
> I want to check what this does, but this all looks like it is doing the right thing.
>
>
>> +	for (i = 0; i < info->nr_segments; i++) {
>> +		if (info->segment[i].mem == (void *)info->rhdr.rel_addr) {
>> +			purgatory_base = (unsigned long)info->segment[i].mem;
>> +			purgatory_len = info->segment[i].memsz;
>> +		}
>> +		mstart = (unsigned long)info->segment[i].mem;
>> +		mend = mstart + info->segment[i].memsz;
>> +		mstart &= ~(SECTION_SIZE - 1);
>> +		while (mstart < mend) {
>> +			create_identity_entry(pgtbl_buf, pgtbl_mem,
>> +					mstart, MMU_FLAGS_NORMAL);
>> +			mstart += SECTION_SIZE;
>> +		}
>> +	}
>


~Pratyush

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