Hi Geoff, On 03/02/2015 06:22 PM, Geoff Levand wrote: > Hi Christoffer, > > On Mon, 2015-03-02 at 14:13 -0800, Christoffer Dall wrote: >> On Wed, Feb 25, 2015 at 02:09:30PM -0800, Geoff Levand wrote: >>> The current hyp-stub vector implementation, which uses x0, is limited >>> to two hyper calls; __hyp_get_vectors and __hyp_set_vectors. To >>> support cpu_soft_restart() we need a third hyper call, one which >>> allows for code to be executed at EL2. My proposed use of the >>> immediate value of the hvc instruction will allow for 2^16 distinct >>> hyper calls. >> >> right, but using x0 allows for 2^64 distinct hypercalls. Just to be >> clear, I'm fine with using immediate field if there are no good reasons >> not to, I was just curious as to what direct benefit it has. After >> thinking about it a bit, from my point of view, the benefit would be the >> clarity that x0 is first argument like a normal procedure call, so no >> need to shift things around. Is this part of the equation or am I >> missing the overall purpose here? > > Yes, in general it will make marshaling of args, etc. easier. Also, > to me, if we are going to change the implementation it seems to be > the most natural way. >From reading the architecture documentation, I too expected the hypervisor call instruction's immediate and the instruction specific syndrome to be used. However I vaguely recall someone pointing out that reading the exception syndrome register and extracting the instruction specific syndrome is bound to take longer than simply using a general purpose register. One might also consider alignment with the SMC Calling Convention document [1], which while originally written for SMC, is also used for HVC by PSCI [2]. 1. http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0028a/index.html 2. http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0022c/index.html Chris -- Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
