Re: [Xen-devel] [PATCH RFC 06/12] xen/x86: populate PVHv2 Dom0 physical memory map

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/08/16 10:19, Roger Pau Monne wrote:
> On Fri, Jul 29, 2016 at 08:04:12PM +0100, Andrew Cooper wrote:
>> On 29/07/16 17:29, Roger Pau Monne wrote:
>>> Craft the Dom0 e820 memory map and populate it.
>>>
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> ---
>>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> ---
>>>  xen/arch/x86/domain_build.c | 199 
>>> ++++++++++++++++++++++++++++++++++++++++++--
>>>  1 file changed, 193 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/domain_build.c b/xen/arch/x86/domain_build.c
>>> index c0ef40f..cb8ecbd 100644
>>> --- a/xen/arch/x86/domain_build.c
>>> +++ b/xen/arch/x86/domain_build.c
>>> @@ -43,6 +43,11 @@ static long __initdata dom0_nrpages;
>>>  static long __initdata dom0_min_nrpages;
>>>  static long __initdata dom0_max_nrpages = LONG_MAX;
>>>  
>>> +/* GFN of the identity map for EPT. */
>>> +#define HVM_IDENT_PT_GFN  0xfeffeu
>> The IDENT_PT is only needed on Gen1 VT-x hardware which doesn't support
>> unrestricted-guest mode.  OTOH, it needs to be matched with VM86_TSS,
>> which is also needed if hardware doesn't support unrestricted-guest. 
>> Also, the IDENT_PT can live anywhere in GFN space.  0xfeffe is just
>> convention for HVM guests as nothing else interesting lives there, but a
>> PVH dom0 will want to ensure it doesn't alias anything interesting it
>> might wish to map.
>>
>> Now I look at it, there is substantial WTF.  The domain builder makes
>> IDENT_PT, but HVMLoader makes VM86_TSS.  I presume this is a historical
>> side effect of IDENT_PT being a prerequisite to even executing
>> hvmloader, while VM86_TSS was only a prerequisite to executing the
>> rombios payload.  Either way, eww :(
>>
>> I think the VM86_TSS setup needs to move to unconditionally being beside
>> IDENT_PT setup in the domain builder, and mirrored here in dom0_hvm()
>> creation.  I don't think it is reasonable to expect an HVMLite payload
>> to set up its own VM86_TSS if it didn't set up IDENT_PT.  (OTOH, the
>> chances of an HVMLite guest ever needing to return to real mode is slim,
>> but in the name of flexibility, it would be nice not to rule it out).
>>
>> Longterm, it would be nice to disentangle the unrestricted-guest support
>> from the main vmx code, and make it able to be compiled out.  There is
>> lots of it, and it all-but-dead on modern hardware.
> Thanks! I didn't know anything about the VM86 TSS stuff, the fact that the 
> identity page tables and the VM86 TSS are set at completely different points 
> makes it quite hard to follow :/.
>
> I've now moved the setup of the VM86 TSS inside the domain builder for both 
> DomU and Dom0.

Thanks.  (Guess who has recently had to delve back into this code^W swamp.)

>
>>> +}
>>> +
>>> +/* Calculate the biggest usable order given a size in bytes. */
>>> +static inline unsigned int get_order(uint64_t size)
>>> +{
>>> +    unsigned int order;
>>> +    uint64_t pg;
>>> +
>>> +    ASSERT((size & ~PAGE_MASK) == 0);
>>> +    pg = PFN_DOWN(size);
>>> +    for ( order = 0; pg >= (1 << (order + 1)); order++ );
>>> +
>>> +    return order;
>>> +}
>> We already have get_order_from_bytes() and get_order_from_pages(), the
>> latter of which looks like it will suit your usecase.
> Not really, or at least they don't do the same as get_order. This function 
> calculates the maximum order you can use so that there are no pages left 
> over, (ie: if you have a size of 3145728bytes (3MiB), this function will 
> return order 9 (2MiB), while the other ones will return order 10 (4MiB)). I 
> don't really understand while other places in code request bigger orders and 
> then free the leftovers, isn't this also causing memory shattering?

Sounds like we want something like get_order_{floor,ceil}() which makes
it obvious which way non-power-of-two get rounded.

>
>> As a TODO item, I really would like to borrow some of the Linux
>> infrastructure to calculate orders of constants at compile time, because
>> a large number of callers of the existing get_order_*() functions are
>> performing unnecessary runtime calculation.  For those which need
>> runtime calculation, some careful use of ffs() would be preferable to a
>> loop.
>>
>>> +
>>> +/* Populate an HVM memory range using the biggest possible order. */
>>> +static void __init hvm_populate_memory_range(struct domain *d, uint64_t 
>>> start,
>>> +                                             uint64_t size)
>>> +{
>>> +    static unsigned int __initdata memflags = MEMF_no_dma|MEMF_exact_node;
>>> +    unsigned int order;
>>> +    struct page_info *page;
>>> +    int rc;
>>> +
>>> +    ASSERT((size & ~PAGE_MASK) == 0 && (start & ~PAGE_MASK) == 0);
>>> +
>>> +    order = MAX_ORDER;
>>> +    while ( size != 0 )
>>> +    {
>>> +        order = min(get_order(size), order);
>>> +        page = alloc_domheap_pages(d, order, memflags);
>>> +        if ( page == NULL )
>>> +        {
>>> +            if ( order == 0 && memflags )
>>> +            {
>>> +                /* Try again without any memflags. */
>>> +                memflags = 0;
>>> +                order = MAX_ORDER;
>>> +                continue;
>>> +            }
>>> +            if ( order == 0 )
>>> +                panic("Unable to allocate memory with order 0!\n");
>>> +            order--;
>>> +            continue;
>>> +        }
>> It would be far more efficient to try and allocate only 1G and 2M
>> blocks.  Most of memory is free at this point, and it would allow the
>> use of HAP superpage mappings, which will be a massive performance boost
>> if they aren't shattered.
> That's what I'm trying to do, but we might have to use pages of lower order 
> when filling the smaller gaps.

As a general principle, we should try not to have any gaps.  This also
extends to guests using more intelligence when deciding now to mutate
its physmap.

>  As an example, this are the stats when 
> building a domain with 6048M of RAM:
>
> (XEN) Memory allocation stats:
> (XEN) Order 18: 5GB
> (XEN) Order 17: 512MB
> (XEN) Order 15: 256MB
> (XEN) Order 14: 128MB
> (XEN) Order 12: 16MB
> (XEN) Order 10: 8MB
> (XEN) Order  9: 4MB
> (XEN) Order  8: 2MB
> (XEN) Order  7: 1MB
> (XEN) Order  6: 512KB
> (XEN) Order  5: 256KB
> (XEN) Order  4: 128KB
> (XEN) Order  3: 64KB
> (XEN) Order  2: 32KB
> (XEN) Order  1: 16KB
> (XEN) Order  0: 4KB
>
> IMHO, they are quite good.

What are the RAM characteristics of the host?  Do you have any idea what
the hap superpage characteristics are like after the guest has booted?

In a case like this, I think it would be entirely reasonable to round up
to the nearest 2MB, and avoid all of those small page mappings.

~Andrew

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