• Sean Christopherson's avatar
    KVM: VMX: Leave preemption timer running when it's disabled · 804939ea
    Sean Christopherson authored
    
    
    VMWRITEs to the major VMCS controls, pin controls included, are
    deceptively expensive.  CPUs with VMCS caching (Westmere and later) also
    optimize away consistency checks on VM-Entry, i.e. skip consistency
    checks if the relevant fields have not changed since the last successful
    VM-Entry (of the cached VMCS).  Because uops are a precious commodity,
    uCode's dirty VMCS field tracking isn't as precise as software would
    prefer.  Notably, writing any of the major VMCS fields effectively marks
    the entire VMCS dirty, i.e. causes the next VM-Entry to perform all
    consistency checks, which consumes several hundred cycles.
    
    As it pertains to KVM, toggling PIN_BASED_VMX_PREEMPTION_TIMER more than
    doubles the latency of the next VM-Entry (and again when/if the flag is
    toggled back).  In a non-nested scenario, running a "standard" guest
    with the preemption timer enabled, toggling the timer flag is uncommon
    but not rare, e.g. roughly 1 in 10 entries.  Disabling the preemption
    timer can change these numbers due to its use for "immediate exits",
    even when explicitly disabled by userspace.
    
    Nested virtualization in particular is painful, as the timer flag is set
    for the majority of VM-Enters, but prepare_vmcs02() initializes vmcs02's
    pin controls to *clear* the flag since its the timer's final state isn't
    known until vmx_vcpu_run().  I.e. the majority of nested VM-Enters end
    up unnecessarily writing pin controls *twice*.
    
    Rather than toggle the timer flag in pin controls, set the timer value
    itself to the largest allowed value to put it into a "soft disabled"
    state, and ignore any spurious preemption timer exits.
    
    Sadly, the timer is a 32-bit value and so theoretically it can fire
    before the head death of the universe, i.e. spurious exits are possible.
    But because KVM does *not* save the timer value on VM-Exit and because
    the timer runs at a slower rate than the TSC, the maximuma timer value
    is still sufficiently large for KVM's purposes.  E.g. on a modern CPU
    with a timer that runs at 1/32 the frequency of a 2.4ghz constant-rate
    TSC, the timer will fire after ~55 seconds of *uninterrupted* guest
    execution.  In other words, spurious VM-Exits are effectively only
    possible if the host is completely tickless on the logical CPU, the
    guest is not using the preemption timer, and the guest is not generating
    VM-Exits for any other reason.
    
    To be safe from bad/weird hardware, disable the preemption timer if its
    maximum delay is less than ten seconds.  Ten seconds is mostly arbitrary
    and was selected in no small part because it's a nice round number.
    For simplicity and paranoia, fall back to __kvm_request_immediate_exit()
    if the preemption timer is disabled by KVM or userspace.  Previously
    KVM continued to use the preemption timer to force immediate exits even
    when the timer was disabled by userspace.  Now that KVM leaves the timer
    running instead of truly disabling it, allow userspace to kill it
    entirely in the unlikely event the timer (or KVM) malfunctions.
    Signed-off-by: default avatarSean Christopherson <sean.j.christopherson@intel.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    804939ea
vmcs.h 3.57 KB