Re: [PATCH RFC 2/5] KVM: x86: introduce KVM_MEM_ALLONES memory

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> On Thu, May 14, 2020 at 08:05:37PM +0200, Vitaly Kuznetsov wrote:
>> PCIe config space can (depending on the configuration) be quite big but
>> usually is sparsely populated. Guest may scan it by accessing individual
>> device's page which, when device is missing, is supposed to have 'pci
>> holes' semantics: reads return '0xff' and writes get discarded. Currently,
>> userspace has to allocate real memory for these holes and fill them with
>> '0xff'. Moreover, different VMs usually require different memory.
>> 
>> The idea behind the feature introduced by this patch is: let's have a
>> single read-only page filled with '0xff' in KVM and map it to all such
>> PCI holes in all VMs. This will free userspace of obligation to allocate
>> real memory. Later, this will also allow us to speed up access to these
>> holes as we can aggressively map the whole slot upon first fault.
>> 
>> Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> ---
>>  Documentation/virt/kvm/api.rst  | 22 ++++++---
>>  arch/x86/include/uapi/asm/kvm.h |  1 +
>>  arch/x86/kvm/x86.c              |  9 ++--
>>  include/linux/kvm_host.h        | 15 ++++++-
>>  include/uapi/linux/kvm.h        |  2 +
>>  virt/kvm/kvm_main.c             | 79 +++++++++++++++++++++++++++++++--
>>  6 files changed, 113 insertions(+), 15 deletions(-)
>> 
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index d871dacb984e..2b87d588a7e0 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -1236,7 +1236,8 @@ yet and must be cleared on entry.
>>  
>>    /* for kvm_memory_region::flags */
>>    #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
>> -  #define KVM_MEM_READONLY	(1UL << 1)
>> +  #define KVM_MEM_READONLY		(1UL << 1)
>> +  #define KVM_MEM_ALLONES		(1UL << 2)
>
> Why not call this KVM_MEM_PCI_HOLE or something else that better conveys
> that this is memslot is intended to emulate PCI master abort semantics?
>

Becuase there's always hope this can be usefult for something else but
PCI? :-) Actually, I was thinking about generalizing this a little bit
to something like KVM_MEM_CONSTANT with a way to set the pattern but I'm
failing to see any need for anything but all-ones or all-zeroes. Maybe
other-than-x86 architectures have some needs?

I'm definitely fine with renaming this to KVM_MEM_PCI_HOLE.

-- 
Vitaly