[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v6 4/6] xen/arm: vgic: Optimize the way to store the target vCPU in the rank
On Wed, 2015-11-18 at 16:42 +0000, Julien Grall wrote: > Xen is currently directly storing the value of GICD_ITARGETSR register > (for GICv2) and GICD_IROUTER (for GICv3) in the rank. This makes the > emulation of the registers access very simple but makes the code to get > the target vCPU for a given vIRQ more complex. > > While the target vCPU of an vIRQ is retrieved every time an vIRQ is > injected to the guest, the access to the register occurs less often. > > So the data structure should be optimized for the most common case > rather than the inverse. > > This patch introduces the usage of an array to store the target vCPU for > every interrupt in the rank. This will make the code to get the target > very quick. The emulation code will now have to generate the > GICD_ITARGETSR > and GICD_IROUTER register for read access and split it to store in a > convenient way. > > With the new way to store the target vCPU, the structure vgic_irq_rank > is shrunk down from 320 bytes to 92 bytes. This is saving about 228 > bytes of memory allocated separately per vCPU. > > Note that with these changes, any read to those register will list only > the target vCPU used by Xen. As the spec is not clear whether this is a > valid choice or not, OSes which have a different interpretation of the > spec (i.e OSes which perform read-modify-write operations on these > registers) may not boot anymore on Xen. Although, I think this is fair > trade between memory usage in Xen (1KB less on a domain using 4 vCPUs > with no SPIs) and a strict interpretation of the spec (though all the > cases are not clearly defined). > > Furthermore, the implementation of the callback get_target_vcpu is now > exactly the same. Consolidate the implementation in the common vGIC code > and drop the callback. > > Finally take the opportunity to fix coding style and replace "irq" by > "virq" to make clear that we are dealing with virtual IRQ in section we > are modifying. > > Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx> I have one clarifying question, which may or may not be worth a followup: > + * Fetch an ITARGETSR register based on the offset from ITARGETSR0. Is the offset here in terms of bytes or in terms of entire ITARGETSR<n> registers (i.e. 4 bytes)? Might be worth clarifying the comment? Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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