Re: [PATCH v2 3/7] x86/sev: add support for RMPOPT instruction

[Kalra, Ashish]

An update on performance data: 

> 
> RMPOPT after SNP guest shutdown:
> ...
> [  298.746893] SEV-SNP: RMPOPT max. CPU cycles 248083620
> [  298.746898] SEV-SNP: RMPOPT min. CPU cycles 60
> [  298.746900] SEV-SNP: RMPOPT average cycles 127859
> 
> 

A single RMPOPT instruction should not be taking 248M cycles, so i looked at
my performance measurement code : 

I was not disabling interrupts around my measurement code, so probably this 
measurement code was getting interrupted/preempted and causing this discrepancy: 

I am now measuring with interrupts disabled around this code: 

static void rmpopt(void *val)
{
        bool optimized;
        u64 start, end;

        local_irq_disable();
        start = rdtsc_ordered();

        asm volatile(".byte 0xf2, 0x0f, 0x01, 0xfc"
                     : "=@ccc" (optimized)
                     : "a" ((u64)val & PUD_MASK), "c" ((u64)val & 0x1)
                     : "memory", "cc");

        end = rdtsc_ordered();
        local_irq_enable();

	total_cycles += (end - start);
        ++iteration;

        if ((end - start) > largest_cycle_rmpopt) {
                pr_info("RMPOPT max cycle on cpu %d, addr 0x%llx, cycles %llu, prev largest %llu\n",
                                smp_processor_id(), ((u64)val & PUD_MASK), end - start, largest_cycle_rmpopt);
                largest_cycle_rmpopt = end - start;
        }
...
...

But, the following is interesting, if I invoke rmpopt() using smp_call_on_cpu() which issues
RMPOPT on each CPU serially compared to using on_each_cpu_mask() above which will execute rmpopt()
function and RMPOPT instruction in parallel on multiple CPUs (by sending IPIs in parallel),
I observe a significant difference and improvement in "individual" RMPOPT instruction performance: 

rmpopt() executing serially using smp_call_on_cpu(): 

[  244.518677] SEV-SNP: RMPOPT instruction cycles 3300
[  244.518716] SEV-SNP: RMPOPT instruction cycles 2840
[  244.518758] SEV-SNP: RMPOPT instruction cycles 3260
[  244.518800] SEV-SNP: RMPOPT instruction cycles 3640
[  244.518838] SEV-SNP: RMPOPT instruction cycles 1980
[  244.518878] SEV-SNP: RMPOPT instruction cycles 3420
[  244.518919] SEV-SNP: RMPOPT instruction cycles 3620
[  244.518958] SEV-SNP: RMPOPT instruction cycles 3120
[  244.518997] SEV-SNP: RMPOPT instruction cycles 2160
[  244.519038] SEV-SNP: RMPOPT instruction cycles 3040
[  244.519078] SEV-SNP: RMPOPT instruction cycles 3700
[  244.519119] SEV-SNP: RMPOPT instruction cycles 3960
[  244.519158] SEV-SNP: RMPOPT instruction cycles 3420
[  244.519211] SEV-SNP: RMPOPT instruction cycles 5080
[  244.519254] SEV-SNP: RMPOPT instruction cycles 3000
[  244.519295] SEV-SNP: RMPOPT instruction cycles 3420
[  244.527150] SEV-SNP: RMPOPT max cycle on cpu 256, addr 0x40000000, cycles 34680, prev largest 22100
[  244.529622] SEV-SNP: RMPOPT max cycle on cpu 320, addr 0x40000000, cycles 36800, prev largest 34680
[  244.559314] SEV-SNP: RMPOPT max cycle on cpu 256, addr 0x80000000, cycles 39740, prev largest 36800
[  244.561718] SEV-SNP: RMPOPT max cycle on cpu 320, addr 0x80000000, cycles 41840, prev largest 39740
[  244.562837] SEV-SNP: RMPOPT max cycle on cpu 352, addr 0x80000000, cycles 42160, prev largest 41840
[  244.886705] SEV-SNP: RMPOPT max cycle on cpu 384, addr 0x300000000, cycles 42300, prev largest 42160
[  247.701377] SEV-SNP: RMPOPT max cycle on cpu 384, addr 0x1980000000, cycles 42400, prev largest 42300
[  250.322355] SEV-SNP: RMPOPT max cycle on cpu 384, addr 0x2ec0000000, cycles 42420, prev largest 42400
[  250.755457] SEV-SNP: RMPOPT max cycle on cpu 384, addr 0x3240000000, cycles 42540, prev largest 42420
[  264.271293] SEV-SNP: RMPOPT max cycle on cpu 32, addr 0xa040000000, cycles 50400, prev largest 42540
[  264.333739] SEV-SNP: RMPOPT max cycle on cpu 32, addr 0xa0c0000000, cycles 50940, prev largest 50400
[  264.395521] SEV-SNP: RMPOPT max cycle on cpu 32, addr 0xa140000000, cycles 51240, prev largest 50940
[  264.733133] SEV-SNP: RMPOPT max cycle on cpu 32, addr 0xa400000000, cycles 51480, prev largest 51240
[  269.500891] SEV-SNP: RMPOPT max cycle on cpu 0, addr 0xcac0000000, cycles 66080, prev largest 51480
[  273.507009] SEV-SNP: RMPOPT max cycle on cpu 320, addr 0xeb40000000, cycles 83680, prev largest 66080
[  276.435091] SEV-SNP: RMPOPT largest cycles 83680
[  276.435096] SEV-SNP: RMPOPT smallest cycles 60
[  276.435097] SEV-SNP: RMPOPT average cycles 5658
[  276.435098] SEV-SNP: RMPOPT cycles taken for physical address range 0x0000000000000000 - 0x0000010380000000 on all cpus 63815935380 cycles

Compare this to executing rmpopt() in parallel:

[ 1238.809183] SEV-SNP: RMPOPT average cycles 114372


So, looks like executing RMPOPT in parallel is causing performance degradation, which we will investigate. 

But, these are the performance numbers you should be considering : 

RMPOPT during boot: 

[   49.913402] SEV-SNP: RMPOPT largest cycles 1143020
[   49.913407] SEV-SNP: RMPOPT smallest cycles 60
[   49.913408] SEV-SNP: RMPOPT average cycles 5226


RMPOPT after SNP guest shutdown: 

[  276.435091] SEV-SNP: RMPOPT largest cycles 83680
[  276.435096] SEV-SNP: RMPOPT smallest cycles 60
[  276.435097] SEV-SNP: RMPOPT average cycles 5658


Thanks,
Ashish