[MODERATED] Re: [SUSPECTED SPAM][PATCH v3 2/6] PERFv3

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 08, 2019 at 12:51:01AM +0000, speck for Andrew Cooper wrote:
> On 07/02/2019 23:41, speck for Andi Kleen wrote:
> > This patch assumes that the kernel is using
> > RETPOLINE (or IBRS), otherwise speculative execution could
> > still corrupt counter 3 in very unlikely cases.
> 
> What has the kernel configuration got to do with it?
> 
> It is my understanding that any execution of an XBEGIN instruction, even
> speculatively, even in userspace will result in PCR3 getting modified.
> 
> A CPU either has force abort mode active, or PCR3 can be changed behind
> the kernel's back.

We are executing kernel code; therefore any user RTM will have aborted
and is irrelevant.

So what the kernel does is:

	/*
	 * And as noted; userspace transactions will be aborted by
	 * having entered the kernel. The kernel does not use RTM
	 * itself.
	 */


	/*
	 * stops all counters; irrespective of ucode using PMC3 or not
	 */
	GLOBAL_CTRL = 0;

	/*
	 * program PMC3
	 */
	CTRVAL3 = x;
	EVTSEL3 = y;

	/*
	 * Set the TFA bit to make ucode not touch PMC3; since there has
	 * not been an RTM instruction between GLOBAL_CTRL=0 and here,
	 * PMC3 will still be {x,y} as we just wrote.
	 *
	 * This is what requires RETPOLINE/IBRS; because otherwise
	 * speculation could see a partial kernel instruction that looks
	 * like RTM, which would mess things up.
	 */
	WRMSR(MSR_TFA, 1);

	/*
	 * Let 'er rip.
	 */
	GLOBAL_CTRL = ~0ULL;