[Qemu-devel] kvm decode cache and guest memory

[Xin Tong]

When kvm encounters a trapping instruction, it will vmexits the guest
and vmenters the host. the host then emulates the instruction on the
vm guest register-saving area. I have a few questions regarding this.

1. There is an emulation context

struct x86_emulate_ctxt {
        struct x86_emulate_ops *ops;

        /* Register state before/after emulation. */
        struct kvm_vcpu *vcpu;

        ...

        /* decode cache */
        struct decode_cache decode;
};

What is the decode_cache here and where does it get filled out ?

2. When the host is emulating instructions for the guest, it may need
to access the guest memory, I guess there are really 2 questions here.
      1. is the guest memory physically contiguous ?
       2. how does the host access the guest memory ? there must be
some functions or macros ?

Thanks

Xin