Re: propagating vmgenid outward and upward

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 01, 2022 at 07:37:06PM +0100, Jason A. Donenfeld wrote:
> Hi Michael,
> 
> On Tue, Mar 1, 2022 at 6:17 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > Hmm okay, so it's a performance optimization... some batching then? Do
> > you really need to worry about every packet? Every 64 packets not
> > enough?  Packets are after all queued at NICs etc, and VM fork can
> > happen after they leave wireguard ...
> 
> Unfortunately, yes, this is an "every packet" sort of thing -- if the
> race is to be avoided in a meaningful way. It's really extra bad:
> ChaCha20 and AES-CTR work by xoring a secret stream of bytes with
> plaintext to produce a ciphertext. If you use that same secret stream
> and xor it with a second plaintext and transmit that too, an attacker
> can combine the two different ciphertexts to learn things about the
> original plaintext.
> 
> But, anyway, it seems like the race is here to stay given what we have
> _currently_ available with the virtual hardware. That's why I'm
> focused on trying to get something going that's the least bad with
> what we've currently got, which is racy by design. How vitally
> important is it to have something that doesn't race in the far future?
> I don't know, really. It seems plausible that that ACPI notifier
> triggers so early that nothing else really even has a chance, so the
> race concern is purely theoretical. But I haven't tried to measure
> that so I'm not sure.
> 
> Jason


I got curious, and wrote a dumb benchmark:


#include <stdio.h>
#include <assert.h>
#include <limits.h>
#include <string.h>

struct lng {
	unsigned long long l1;
	unsigned long long l2;
};

struct shrt {
	unsigned long s;
};


struct lng l = { 1, 2 };
struct shrt s = { 3 };

static void test1(volatile struct shrt *sp)
{
	if (sp->s != s.s) {
		printf("short mismatch!\n");
		s.s = sp->s;
	}
}
static void test2(volatile struct lng *lp)
{
	if (lp->l1 != l.l1 || lp->l2 != l.l2) {
		printf("long mismatch!\n");
		l.l1 = lp->l1;
		l.l2 = lp->l2;
	}
}

int main(int argc, char **argv)
{
	volatile struct shrt sv = { 4 };
	volatile struct lng lv = { 5, 6 };

	if (argc > 1) {
		printf("test 1\n");
		for (int i = 0; i < 10000000; ++i) 
			test1(&sv);
	} else {
		printf("test 2\n");
		for (int i = 0; i < 10000000; ++i)
			test2(&lv);
	}
	return 0;
}


Results (built with -O2, nothing fancy):

[mst@tuck ~]$ perf stat -r 1000 ./a.out 1 > /dev/null

 Performance counter stats for './a.out 1' (1000 runs):

              5.12 msec task-clock:u              #    0.945 CPUs utilized            ( +-  0.07% )
                 0      context-switches:u        #    0.000 /sec                   
                 0      cpu-migrations:u          #    0.000 /sec                   
                52      page-faults:u             #   10.016 K/sec                    ( +-  0.07% )
        20,190,800      cycles:u                  #    3.889 GHz                      ( +-  0.01% )
        50,147,371      instructions:u            #    2.48  insn per cycle           ( +-  0.00% )
        20,032,224      branches:u                #    3.858 G/sec                    ( +-  0.00% )
             1,604      branch-misses:u           #    0.01% of all branches          ( +-  0.26% )

        0.00541882 +- 0.00000847 seconds time elapsed  ( +-  0.16% )

[mst@tuck ~]$ perf stat -r 1000 ./a.out > /dev/null

 Performance counter stats for './a.out' (1000 runs):

              7.75 msec task-clock:u              #    0.947 CPUs utilized            ( +-  0.12% )
                 0      context-switches:u        #    0.000 /sec                   
                 0      cpu-migrations:u          #    0.000 /sec                   
                52      page-faults:u             #    6.539 K/sec                    ( +-  0.07% )
        30,205,916      cycles:u                  #    3.798 GHz                      ( +-  0.01% )
        80,147,373      instructions:u            #    2.65  insn per cycle           ( +-  0.00% )
        30,032,227      branches:u                #    3.776 G/sec                    ( +-  0.00% )
             1,621      branch-misses:u           #    0.01% of all branches          ( +-  0.23% )

        0.00817982 +- 0.00000965 seconds time elapsed  ( +-  0.12% )


So yes, the overhead is higher by 50% which seems a lot but it's from a
very small number, so I don't see why it's a show stopper, it's not by a
factor of 10 such that we should sacrifice safety by default. Maybe a
kernel flag that removes the read replacing it with an interrupt will
do.

In other words, premature optimization is the root of all evil.

-- 
MST