On Thu, Sep 15, 2016 at 05:50:37PM +0200, Lluís Vilanova wrote: > Avoids generating TCG code to call guest code tracing events in vCPUs that are > not dynamically tracing that event. > > Currently, events with the 'tcg' property always generate TCG code to trace > that > event at guest code execution time, when their dynamic tracing state is > checked. > > This series adds a performance optimization where TCG code for events with the > 'tcg' and 'vcpu' properties is not generated if the event is dynamically > disabled. This optimization raises two issues: > > * An event can be dynamically disabled/enabled after the corresponding TCG > code > has been generated (i.e., a new TB with the corresponding code should be > used). > > * Each vCPU can have a different dynamic state for the same event (i.e., > tracing > the memory accesses of only one process pinned to a vCPU). > > To handle both issues, this series replicates the shared physical TB cache, > creating a separate physical TB cache for every combination of event states > (those with the 'vcpu' and 'tcg' properties). Then, all vCPUs tracing the same > events will use the same physical TB cache. > > Sharing physical TBs makes this very space efficient (only the physical TB > caches, simple arrays of pointers, are replicated), sharing physical TB caches > maximizes TB reuse across vCPUs whenever possible, and makes dynamic event > state > changes more efficient (simply use a different TB array). > > The physical TB cache array is indexed with the vCPU's trace event state > bitmask. This is simpler and more efficient than emitting TCG code to check if > an event needs tracing; then we should still move the tracing call code to > either a cold path (making tracing performance worse), or leave it inlined > (making non-tracing performance worse). > > It is also more efficient than eliding TCG code only when *zero* vCPUs are > tracing an event, since enabling it on a single vCPU will impact the > performance > of all other vCPUs that are not tracing that event. > > Signed-off-by: Lluís Vilanova <vilan...@ac.upc.edu> > ---
TCG folks? The design of this patch is more related to TCG than tracing since it affects TB caching. Stefan
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