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
