Re: [PATCH v2 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/04/16 12:14, Yu, Zhang wrote:
>
>
> On 4/8/2016 9:33 PM, Andrew Cooper wrote:
>> On 31/03/16 11:53, Yu Zhang wrote:
>>> A new HVMOP - HVMOP_map_mem_type_to_ioreq_server, is added to
>>> let one ioreq server claim/disclaim its responsibility for the
>>> handling of guest pages with p2m type p2m_ioreq_server. Users
>>> of this HVMOP can specify whether the p2m_ioreq_server is supposed
>>> to handle write accesses or read ones or both in a parameter named
>>> flags. For now, we only support one ioreq server for this p2m type,
>>> so once an ioreq server has claimed its ownership, subsequent calls
>>> of the HVMOP_map_mem_type_to_ioreq_server will fail. Users can also
>>> disclaim the ownership of guest ram pages with this p2m type, by
>>> triggering this new HVMOP, with ioreq server id set to the current
>>> owner's and flags parameter set to 0.
>>>
>>> For now, both HVMOP_map_mem_type_to_ioreq_server and p2m_ioreq_server
>>> are only supported for HVMs with HAP enabled.
>>>
>>> Note that flags parameter(if not 0) of this HVMOP only indicates
>>> which kind of memory accesses are to be forwarded to an ioreq server,
>>> it has impact on the access rights of guest ram pages, but are not
>>> the same. Due to hardware limitations, if only write operations are
>>> to be forwarded, read ones will be performed at full speed, with
>>> no hypervisor intervention. But if read ones are to be forwarded to
>>> an ioreq server, writes will inevitably be trapped into hypervisor,
>>> which means significant performance impact.
>>>
>>> Also note that this HVMOP_map_mem_type_to_ioreq_server will not
>>> change the p2m type of any guest ram page, until HVMOP_set_mem_type
>>> is triggered. So normally the steps should be the backend driver
>>> first claims its ownership of guest ram pages with p2m_ioreq_server
>>> type, and then sets the memory type to p2m_ioreq_server for specified
>>> guest ram pages.
>>>
>>> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
>>> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
>>> Cc: Keir Fraser <keir@xen.org>
>>> Cc: Jan Beulich <jbeulich@suse.com>
>>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>>> Cc: George Dunlap <george.dunlap@eu.citrix.com>
>>> Cc: Jun Nakajima <jun.nakajima@intel.com>
>>> Cc: Kevin Tian <kevin.tian@intel.com>
>>> Cc: Tim Deegan <tim@xen.org>
>>> ---
>>>   xen/arch/x86/hvm/emulate.c       | 125
>>> +++++++++++++++++++++++++++++++++++++--
>>>   xen/arch/x86/hvm/hvm.c           |  95 +++++++++++++++++++++++++++--
>>>   xen/arch/x86/mm/hap/nested_hap.c |   2 +-
>>>   xen/arch/x86/mm/p2m-ept.c        |  14 ++++-
>>>   xen/arch/x86/mm/p2m-pt.c         |  25 +++++---
>>>   xen/arch/x86/mm/p2m.c            |  82 +++++++++++++++++++++++++
>>>   xen/arch/x86/mm/shadow/multi.c   |   3 +-
>>>   xen/include/asm-x86/p2m.h        |  36 +++++++++--
>>>   xen/include/public/hvm/hvm_op.h  |  37 ++++++++++++
>>>   9 files changed, 395 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
>>> index ddc8007..77a4793 100644
>>> --- a/xen/arch/x86/hvm/emulate.c
>>> +++ b/xen/arch/x86/hvm/emulate.c
>>> @@ -94,11 +94,69 @@ static const struct hvm_io_handler null_handler = {
>>>       .ops = &null_ops
>>>   };
>>>
>>> +static int mem_read(const struct hvm_io_handler *io_handler,
>>> +                    uint64_t addr,
>>> +                    uint32_t size,
>>> +                    uint64_t *data)
>>> +{
>>> +    struct domain *currd = current->domain;
>>> +    unsigned long gmfn = paddr_to_pfn(addr);
>>> +    unsigned long offset = addr & ~PAGE_MASK;
>>> +    struct page_info *page = get_page_from_gfn(currd, gmfn, NULL,
>>> P2M_UNSHARE);
>>> +    uint8_t *p;
>>> +
>>> +    if ( !page )
>>> +        return X86EMUL_UNHANDLEABLE;
>>> +
>>> +    p = __map_domain_page(page);
>>> +    p += offset;
>>> +    memcpy(data, p, size);
>>
>> What happens when offset + size crosses the page boundary?
>>
>
> The 'size' is set in hvmemul_linear_mmio_access(), to insure offset +
> size will not cross the page boundary.

Ok, in which case please

ASSERT(offset + size < PAGE_SIZE)

So it is absolutely clear in this function that the caller is required
to honour this restriction.

~Andrew

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