• Jeremy Fitzhardinge's avatar
    xen: Core Xen implementation · 5ead97c8
    Jeremy Fitzhardinge authored
    This patch is a rollup of all the core pieces of the Xen
    implementation, including:
     - booting and setup
     - pagetable setup
     - privileged instructions
     - segmentation
     - interrupt flags
     - upcalls
     - multicall batching
    
    BOOTING AND SETUP
    
    The vmlinux image is decorated with ELF notes which tell the Xen
    domain builder what the kernel's requirements are; the domain builder
    then constructs the address space accordingly and starts the kernel.
    
    Xen has its own entrypoint for the kernel (contained in an ELF note).
    The ELF notes are set up by xen-head.S, which is included into head.S.
    In principle it could be linked separately, but it seems to provoke
    lots of binutils bugs.
    
    Because the domain builder starts the kernel in a fairly sane state
    (32-bit protected mode, paging enabled, flat segments set up), there's
    not a lot of setup needed before starting the kernel proper.  The main
    steps are:
      1. Install the Xen paravirt_ops, which is simply a matter of a
         structure assignment.
      2. Set init_mm to use the Xen-supplied pagetables (analogous to the
         head.S generated pagetables in a native boot).
      3. Reserve address space for Xen, since it takes a chunk at the top
         of the address space for its own use.
      4. Call start_kernel()
    
    PAGETABLE SETUP
    
    Once we hit the main kernel boot sequence, it will end up calling back
    via paravirt_ops to set up various pieces of Xen specific state.  One
    of the critical things which requires a bit of extra care is the
    construction of the initial init_mm pagetable.  Because Xen places
    tight constraints on pagetables (an active pagetable must always be
    valid, and must always be mapped read-only to the guest domain), we
    need to be careful when constructing the new pagetable to keep these
    constraints in mind.  It turns out that the easiest way to do this is
    use the initial Xen-provided pagetable as a template, and then just
    insert new mappings for memory where a mapping doesn't already exist.
    
    This means that during pagetable setup, it uses a special version of
    xen_set_pte which ignores any attempt to remap a read-only page as
    read-write (since Xen will map its own initial pagetable as RO), but
    lets other changes to the ptes happen, so that things like NX are set
    properly.
    
    PRIVILEGED INSTRUCTIONS AND SEGMENTATION
    
    When the kernel runs under Xen, it runs in ring 1 rather than ring 0.
    This means that it is more privileged than user-mode in ring 3, but it
    still can't run privileged instructions directly.  Non-performance
    critical instructions are dealt with by taking a privilege exception
    and trapping into the hypervisor and emulating the instruction, but
    more performance-critical instructions have their own specific
    paravirt_ops.  In many cases we can avoid having to do any hypercalls
    for these instructions, or the Xen implementation is quite different
    from the normal native version.
    
    The privileged instructions fall into the broad classes of:
      Segmentation: setting up the GDT and the GDT entries, LDT,
         TLS and so on.  Xen doesn't allow the GDT to be directly
         modified; all GDT updates are done via hypercalls where the new
         entries can be validated.  This is important because Xen uses
         segment limits to prevent the guest kernel from damaging the
         hypervisor itself.
      Traps and exceptions: Xen uses a special format for trap entrypoints,
         so when the kernel wants to set an IDT entry, it needs to be
         converted to the form Xen expects.  Xen sets int 0x80 up specially
         so that the trap goes straight from userspace into the guest kernel
         without going via the hypervisor.  sysenter isn't supported.
      Kernel stack: The esp0 entry is extracted from the tss and provided to
         Xen.
      TLB operations: the various TLB calls are mapped into corresponding
         Xen hypercalls.
      Control registers: all the control registers are privileged.  The most
         important is cr3, which points to the base of the current pagetable,
         and we handle it specially.
    
    Another instruction we treat specially is CPUID, even though its not
    privileged.  We want to control what CPU features are visible to the
    rest of the kernel, and so CPUID ends up going into a paravirt_op.
    Xen implements this mainly to disable the ACPI and APIC subsystems.
    
    INTERRUPT FLAGS
    
    Xen maintains its own separate flag for masking events, which is
    contained within the per-cpu vcpu_info structure.  Because the guest
    kernel runs in ring 1 and not 0, the IF flag in EFLAGS is completely
    ignored (and must be, because even if a guest domain disables
    interrupts for itself, it can't disable them overall).
    
    (A note on terminology: "events" and interrupts are effectively
    synonymous.  However, rather than using an "enable flag", Xen uses a
    "mask flag", which blocks event delivery when it is non-zero.)
    
    There are paravirt_ops for each of cli/sti/save_fl/restore_fl, which
    are implemented to manage the Xen event mask state.  The only thing
    worth noting is that when events are unmasked, we need to explicitly
    see if there's a pending event and call into the hypervisor to make
    sure it gets delivered.
    
    UPCALLS
    
    Xen needs a couple of upcall (or callback) functions to be implemented
    by each guest.  One is the event upcalls, which is how events
    (interrupts, effectively) are delivered to the guests.  The other is
    the failsafe callback, which is used to report errors in either
    reloading a segment register, or caused by iret.  These are
    implemented in i386/kernel/entry.S so they can jump into the normal
    iret_exc path when necessary.
    
    MULTICALL BATCHING
    
    Xen provides a multicall mechanism, which allows multiple hypercalls
    to be issued at once in order to mitigate the cost of trapping into
    the hypervisor.  This is particularly useful for context switches,
    since the 4-5 hypercalls they would normally need (reload cr3, update
    TLS, maybe update LDT) can be reduced to one.  This patch implements a
    generic batching mechanism for hypercalls, which gets used in many
    places in the Xen code.
    Signed-off-by: default avatarJeremy Fitzhardinge <jeremy@xensource.com>
    Signed-off-by: default avatarChris Wright <chrisw@sous-sol.org>
    Cc: Ian Pratt <ian.pratt@xensource.com>
    Cc: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
    Cc: Adrian Bunk <bunk@stusta.de>
    5ead97c8