On Sun Jun 27 around 19:33:52 EST 2010 Alexander Graf wrote: > Am 27.06.2010 um 10:14 schrieb Avi Kivity <avi at redhat.com>: > > On 06/26/2010 02:25 AM, Alexander Graf wrote: > > > + > > > +PPC hypercalls > > > +============== > > > + > > > +The only viable ways to reliably get from guest context to host > > > context are: > > > + > > > + 1) Call an invalid instruction > > > + 2) Call the "sc" instruction with a parameter to "sc" > > > + 3) Call the "sc" instruction with parameters in GPRs > > > + > > > +Method 1 is always a bad idea. Invalid instructions can be > > > replaced later on > > > +by valid instructions, rendering the interface broken. > > > + > > > +Method 2 also has downfalls. If the parameter to "sc" is != 0 the > > > spec is > > > +rather unclear if the sc is targeted directly for the hypervisor > > > or the > > > +supervisor. It would also require that we read the syscall issuing > > > instruction > > > +every time a syscall is issued, slowing down guest syscalls. > > > + It goes to the hypervisor, and it would require the hypervisor to return to the supervisor, but I believe it just returns to the user with permission denied. > > > +Method 3 is what KVM uses. We pass magic constants > > > (KVM_SC_MAGIC_R3 and > > > +KVM_SC_MAGIC_R4) in r3 and r4 respectively. If a syscall > > > instruction with these > > > +magic values arrives from the guest's kernel mode, we take the > > > syscall as a > > > +hypercall. > > > > > > > Is there any chance a normal syscall will have those values in r3 > > and r4? > > r3 is the syscall number. So as long as the guest doesn't reuse that > value, we're safe. Since in general syscall numbers are not randomly > scattered throughout the number range, we should be ok here. > No, r0 has the system call number. Registers 3 and 4 are the first 2 args in c abi (or first 64 bit arg in 32 bit c abi), but the linux syscall abi special. (In addition, it returns success or failure in cr0). > > > > If so, maybe it's better to use pc as they key for hypercalls. Let > > the guest designate one instruction address as the hypercall call > > point; kvm can easily check it and reflect it back to the guest if > > it doesn't match. > > > > You mean the guest would tell the hv where the hypercall lies? That > would require a hypercall, no? Defining it statically is tricky. I > want to PV'nize osx using a kernel module later, so I don't have > control over the physical layout. > > > Is it valid and useful to issue sc from privileged mode anyway, > > except for calling the hypervisor? > > Same as a syscall on x86 really. The kernel can and does issue > syscalls within itself. > > I don't believe we support the kernel actually doing a syscall to itself anymore, at least on powerpc. The callers call the underlying system call function, or kernel_thread. That said, I would suggest we allocate a syscall number for this, as it would document the usage. (In additon to 0..nr_syscalls - 1 we have 0x1ebe in use). Also, is there any desire to nest such emulation? milton -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
