Re: VM Memory Map

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 11, 2023, Yahya Sohail wrote:
> Hi,
> 
> Accesses to certain memory addresses by the guest trigger a KVM_EXIT_MMIO. I
> can't seem to find a memory map in the documentation or source that
> describes exactly which addresses are real memory and which addresses are
> MMIO addresses (on x86, if that matters). Is there any such documentation or
> a listing in the source?
> 
> Is there any way to configure which addresses are MMIO? I hoped that mapping
> memory to MMIO address regions with the KVM_SET_USER_MEMORY_REGION ioctl
> would allow me to use them as memory, but that didn't work. The only ioctls
> that seem relevant to MMIO are KVM_(UN)REGISTER_COALESCED_MMIO, but those
> only allow coalescing MMIO exits, not changing which addresses cause them.

KVM_EXIT_MMIO is for *emulated* MMIO, and is triggered by a guest access to
non-existent memory from the guest's (and KVM's) perspective.  Specifically, if
the guest accesses a GPA that is not covered by a memslot.

Mapping a "real" host MMIO address into a guest via KVM_SET_USER_MEMORY_REGION
will not generate KVM_EXIT_MMIO, as KVM will simply map the "real" MMIO directly
into the guest.

There is no KVM documentation of a memory map or real vs. emulated addresses because
what is real and what is emulated is completely userspace defined (except for the
local APIC and/or I/O APIC if userspace enables KVM's "in-kernel" APIC emulation).