On 23/02/21 18:06, Laszlo Ersek wrote:
On 02/23/21 08:45, Paolo Bonzini wrote:
On 22/02/21 15:53, Laszlo Ersek wrote:
+
+ if (mCpuHotEjectData != NULL) {
+ CPU_HOT_EJECT_HANDLER Handler;
+
+ Handler = mCpuHotEjectData->Handler;
This patch looks otherwise OK to me, but:
In patch v8 08/10, we have a ReleaseMemoryFence(). (For now, it is only
expressed as a MemoryFence() call; we'll make that more precise later.)
(1) I think that should be paired with an AcquireMemoryFence() call,
just before loading "mCpuHotEjectData->Handler" above -- for now, also
expressed as a MemoryFence() call only.
In Linux terms, there is a control dependency here. However, it should
at least be a separate statement to load mCpuHotEjectData (which from my
EDK2 reminiscences should be a global) into a local variable. So
EjectData = mCPUHotEjectData;
// Optional AcquireMemoryFence here
if (EjectData != NULL) {
CPU_HOT_EJECT_HANDLER Handler;
Handler = EjectData->Handler;
if (Handler != NULL) {
Handler (CpuIndex);
}
}
Yes, "mCPUHotEjectData" is a global.
"mCpuHotEjectData" itself is set up on the BSP (from the entry point
function of the PiSmmCpuSmmDxe driver), before any other APs have a
chance to execute any SMM-related code at all. Furthermore, once set up,
mCpuHotEjectData never changes -- it remains set to a particular
non-NULL value forever, or it remains NULL forever. (The latter case
applies when the possible CPU count is 1; IOW, then there is no AP at all.)
Ok, that's what I was missing. However, your code below has *two* loads
of mCpuHotEjectData and the fence would have to go after the second
(between the load of mCpuHotEjectData and the load of the Handler
field). Therefore I would still use a local variable even if you decide
to put the fence inside the "if", which I agree is the clearest.
Paolo
Because of that, I thought that the first comparison (mCpuHotEjectData
!= NULL) would not need any fence -- I thought it was similar to a
userspace program that (a) set a global variable in the "main" thread,
before calling pthread_create(), (b) treated the global variable as a
constant, ever after (meaning all threads).
However, mCpuHotEjectData->Handler is changed regularly (modified by the
BSP, and read "later" by all processors). That's why I thought the
acquire fence was needed in the following location:
if (mCpuHotEjectData != NULL) {
CPU_HOT_EJECT_HANDLER Handler;
//
// HERE -- AcquireMemoryFence()
//
Handler = mCpuHotEjectData->Handler;
if (Handler != NULL) {
Handler (CpuIndex);
}
}
Thanks!
Laszlo
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