On Mon, Sep 20, 2021 at 10:47:08PM -0700, Lucas De Marchi wrote:
On Wed, Sep 15, 2021 at 12:29:12PM -0700, John Harrison wrote:
On 9/15/2021 12:24, Belgaumkar, Vinay wrote:
On 9/14/2021 12:51 PM, Lucas De Marchi wrote:
The clflush calls here aren't doing anything since we are not writting something and flushing the cache lines to be visible to GuC. Here the intention seems to be to make sure whatever GuC has written is visible to the CPU before we read them. However a clflush from the CPU side is the wrong instruction to use.
Is there a right instruction to use? Either we need to verify that no
how can there be a right instruction? If the GuC needs to flush, then the GuC needs to do it, nothing to be done by the CPU.
Flushing the CPU cache line here is doing nothing to guarantee that what was written by GuC hit the memory and we are reading it. Not sure why it was actually added, but since it was added by Vinay and he reviewed this patch, I'm assuming he also agrees
clflush == writeback + invalidate. The invalidate is the important part when the CPU has to read something written by something else that's not cache coherent.
Now, I have no idea if the guc has its own (CPU invisible) caches or not. If it does then it will need to trigger a writeback. But regardless, if the guc bypasses the CPU caches the CPU will need to invalidate before it reads anything in case it has stale data sitting in its cache.