On Tue, Jan 27, 2015 at 05:39:47PM +0000, Catalin Marinas wrote: > On Sat, Jan 17, 2015 at 12:23:34AM +0000, Geoff Levand wrote: > > diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h > > index 99c319c..4f23a48 100644 > > --- a/arch/arm64/include/asm/virt.h > > +++ b/arch/arm64/include/asm/virt.h > > @@ -41,6 +41,19 @@ > > > > #define HVC_CALL_HYP 3 > > > > +/* > > + * HVC_CALL_FUNC - Execute a function at EL2. > > + * > > + * @x0: Physical address of the function to be executed. > > + * @x1: Passed as the first argument to the function. > > + * @x2: Passed as the second argument to the function. > > + * @x3: Passed as the third argument to the function. > > + * > > + * The called function must preserve the contents of register x18. > > Can you pick a register that's normally callee saved? We're in the hyp-stub, so we don't have a stack in EL2. Therefore we can't stack any of the existing callee-saved register values in order to be able to use them. One way to avoid that would be to have asm block which issues the HVC at EL1 stack/unstack the LR around the HVC. Then we're free to corrupt the LR at EL2 in order to call the provided function. [...] > > +1: cmp x18, #HVC_CALL_FUNC > > + b.ne 2f > > + mov x18, lr > > + mov lr, x0 > > + mov x0, x1 > > + mov x1, x2 > > + mov x2, x3 > > + blr lr > > + mov lr, x18 > > > > 2: eret > > ENDPROC(el1_sync) > > What is the calling convention for this HVC? You mentioned x18 above but > what about other registers that the called function may corrupt (x18 is > a temporary register, so it's not expected to be callee saved). Other than x18, the usual PCS rules apply here. We don't have a stack, so the function we call can't make a nested call to anything else. Mark.
