Re: [Xen-devel] [v8][PATCH 07/16] hvmloader/e820: construct guest e820 table

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 07/16/2015 04:29 PM, Chen, Tiejun wrote:
> 
> 
> On 2015/7/16 23:16, George Dunlap wrote:
>> On 07/16/2015 04:04 PM, Chen, Tiejun wrote:
>>>> Yes, sorry, add_high_mem will be the size of memory *relocated*, not
>>>> the actual end of it (unless, as you say, the original highmem region
>>>> didn't exist).
>>>>
>>>> What I really meant was that either way, after adjusting the highmem
>>>> region in the e820, the end of that region should correspond to
>>>> hvm_info->high_mem_pgend.
>>>>
>>>> What about something like this?
>>>> ---
>>>>       /*
>>>>        * And then we also need to adjust highmem.
>>>>        */
>>>>       if ( add_high_mem )
>>>>       {
>>>>           /*
>>>>            * Modify the existing highmem region if it exists
>>>>            */
>>>>           for ( i = 0; i < nr; i++ )
>>>>           {
>>>>               if ( e820[i].type == E820_RAM &&
>>>>                    e820[i].addr == (1ull << 32))
>>>>               {
>>>>                   e820[i].size += add_high_mem;
>>>>                   break;
>>>>               }
>>>>           }
>>>>
>>>>           /*
>>>>            * If we didn't find a highmem region, make one
>>>>            */
>>>>           if ( i == nr )
>>>>           {
>>>>               e820[nr].addr = ((uint64_t)1 << 32);
>>>>               e820[nr].size = e820[nr].addr + add_high_mem;
>>>>               e820[nr].type = E820_RAM;
>>>>               nr++;
>>>>           }
>>>>
>>>>           /*
>>>>            * Either way, at this point i points to the entry containing
>>>>            * highmem.  Compare it to what's in hvm_info as a sanity
>>>>            * check.
>>>>            */
>>>>           BUG_ON(e820[i].addr+e820[i].size !=
>>>>                  ((uint64_t)hvm_info->high_mem_pgend << PAGE_SHIFT));
>>>>       }
>>>>
>>>
>>> Looks really better.
>>>
>>> I just introduce a little change based on yours, and I post this as a
>>> whole,
>>>
>>> diff --git a/tools/firmware/hvmloader/e820.c
>>> b/tools/firmware/hvmloader/e820.c
>>> index 7a414ab..8c9b01f 100644
>>> --- a/tools/firmware/hvmloader/e820.c
>>> +++ b/tools/firmware/hvmloader/e820.c
>>> @@ -105,7 +105,10 @@ int build_e820_table(struct e820entry *e820,
>>>                        unsigned int lowmem_reserved_base,
>>>                        unsigned int bios_image_base)
>>>   {
>>> -    unsigned int nr = 0;
>>> +    unsigned int nr = 0, i, j;
>>> +    uint32_t low_mem_end = hvm_info->low_mem_pgend << PAGE_SHIFT;
>>> +    uint64_t high_mem_end = (uint64_t)hvm_info->high_mem_pgend <<
>>> PAGE_SHIFT;
>>> +    uint64_t add_high_mem = 0;
>>>
>>>       if ( !lowmem_reserved_base )
>>>               lowmem_reserved_base = 0xA0000;
>>> @@ -149,13 +152,6 @@ int build_e820_table(struct e820entry *e820,
>>>       e820[nr].type = E820_RESERVED;
>>>       nr++;
>>>
>>> -    /* Low RAM goes here. Reserve space for special pages. */
>>> -    BUG_ON((hvm_info->low_mem_pgend << PAGE_SHIFT) < (2u << 20));
>>> -    e820[nr].addr = 0x100000;
>>> -    e820[nr].size = (hvm_info->low_mem_pgend << PAGE_SHIFT) -
>>> e820[nr].addr;
>>> -    e820[nr].type = E820_RAM;
>>> -    nr++;
>>> -
>>>       /*
>>>        * Explicitly reserve space for special pages.
>>>        * This space starts at RESERVED_MEMBASE an extends to cover
>>> various
>>> @@ -191,16 +187,91 @@ int build_e820_table(struct e820entry *e820,
>>>           nr++;
>>>       }
>>>
>>> -
>>> -    if ( hvm_info->high_mem_pgend )
>>> +    /*
>>> +     * Construct E820 table according to recorded memory map.
>>> +     *
>>> +     * The memory map created by toolstack may include,
>>> +     *
>>> +     * #1. Low memory region
>>> +     *
>>> +     * Low RAM starts at least from 1M to make sure all standard
>>> regions
>>> +     * of the PC memory map, like BIOS, VGA memory-mapped I/O and
>>> vgabios,
>>> +     * have enough space.
>>> +     *
>>> +     * #2. Reserved regions if they exist
>>> +     *
>>> +     * #3. High memory region if it exists
>>> +     */
>>> +    for ( i = 0; i < memory_map.nr_map; i++ )
>>>       {
>>> -        e820[nr].addr = ((uint64_t)1 << 32);
>>> -        e820[nr].size =
>>> -            ((uint64_t)hvm_info->high_mem_pgend << PAGE_SHIFT) -
>>> e820[nr].addr;
>>> -        e820[nr].type = E820_RAM;
>>> +        e820[nr] = memory_map.map[i];
>>>           nr++;
>>>       }
>>>
>>> +    /* Low RAM goes here. Reserve space for special pages. */
>>> +    BUG_ON(low_mem_end < (2u << 20));
>>> +
>>> +    /*
>>> +     * Its possible to relocate RAM to allocate sufficient MMIO
>>> previously
>>> +     * so low_mem_pgend would be changed over there. And here
>>> memory_map[]
>>> +     * records the original low/high memory, so if low_mem_end is less
>>> than
>>> +     * the original we need to revise low/high memory range in e820.
>>> +     */
>>> +    for ( i = 0; i < nr; i++ )
>>> +    {
>>> +        uint64_t end = e820[i].addr + e820[i].size;
>>> +        if ( e820[i].type == E820_RAM &&
>>> +             low_mem_end > e820[i].addr && low_mem_end < end )
>>> +        {
>>> +            add_high_mem = end - low_mem_end;
>>> +            e820[i].size = low_mem_end - e820[i].addr;
>>> +        }
>>> +    }
>>> +
>>> +    /*
>>> +     * And then we also need to adjust highmem.
>>> +     */
>>> +    if ( add_high_mem )
>>> +    {
>>> +        /* Modify the existing highmem region if it exists. */
>>> +        for ( i = 0; i < nr; i++ )
>>> +        {
>>> +            if ( e820[i].type == E820_RAM &&
>>> +                 e820[i].addr == ((uint64_t)1 << 32))
>>> +            {
>>> +                e820[i].size += add_high_mem;
>>> +                break;
>>> +            }
>>> +        }
>>> +
>>> +        /* If there was no highmem region, just create one. */
>>> +        if ( i == nr )
>>> +        {
>>> +            e820[nr].addr = ((uint64_t)1 << 32);
>>> +            e820[nr].size = high_mem_end  - e820[nr].addr;
>>> +            e820[nr].type = E820_RAM;
>>> +            nr++;
>>> +        }
>>> +
>>> +        /* A sanity check if high memory is broken. */
>>> +        BUG_ON( high_mem_end != e820[i].addr + e820[i].size);
>>
>> The reason I wrote it the way I did was so that we would cross-check our
>> lowmem adjustments (via add_high_mem) with the value in hvm_info in
>> *both cases*.
>>
>> In the code above, you'll get the sanity check if we modify an existing
>> e820 entry; but if we create a new entry, then we don't check to make
>> sure that the amount we removed from the lowmem entry equals the amount
>> we added to the highmem entry.
> 
> Are you saying the following two cases are not same?
> 
> uint64_t high_mem_end = (uint64_t)hvm_info->high_mem_pgend << PAGE_SHIFT;
> BUG_ON( high_mem_end != e820[i].addr + e820[i].size);
> vs.
> BUG_ON(e820[i].addr+e820[i].size != ((uint64_t)hvm_info->high_mem_pgend
> << PAGE_SHIFT));
> 
> Why? Note hvm_info->high_mem_pgend don't change while build e820 table.
> 
> Honestly I didn't try to change that point but maybe I'm missing something?

Yes, you are missing something. :-)  I told you exactly what I wanted
changed and what I said could remain the same:

>> By all means, calculate high_mem_end so it's easier to read.  But then,
>> when creating a new region, set e820[nr].size = add_high_mem, so that
>> the BUG_ON() that follows actually checks something useful.

Just to be clear, I want the second if() statement to look like this:

>>> +        if ( i == nr )
>>> +        {
>>> +            e820[nr].addr = ((uint64_t)1 << 32);
>>> +            e820[nr].size = add_high_mem;
>>> +            e820[nr].type = E820_RAM;
>>> +            nr++;
>>> +        }

Think about why and maybe that will help you understand what I'm talking
about.

 -George

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