Re: [RFC][PATCH] KVM: SVM: Sync g_pat with guest-written PAT value

["Radim Krčmář" <rkrcmar@redhat.com>]

2015-04-20 18:14+0200, Radim Krčmář:
> Tested-by: Radim Krčmář <rkrcmar@redhat.com>

Uncached accesses were roughly 20x slower.
In case anyone wanted to reproduce, I used this as a kvm-unit-test:

---
#include "processor.h"

#define NR_TOP_LOOPS 24
#define NR_MEM_LOOPS 10
#define MEM_ELEMENTS 1024

static volatile u64 pat_test_memory[MEM_ELEMENTS];

static void flush_tlb(void)
{
	write_cr3(read_cr3());
}

static void set_pat(u64 val)
{
	wrmsr(0x277, val);
	flush_tlb();

}

static u64 time_memory_accesses(void)
{
	u64 tsc_before = rdtsc();

	for (unsigned loop = 0; loop < NR_MEM_LOOPS; loop++)
		for (unsigned i = 0; i < MEM_ELEMENTS; i++)
			pat_test_memory[i]++;

	return rdtsc() - tsc_before;
}

int main(int argc, char **argv)
{
	unsigned error = 0;

	for (unsigned loop = 0; loop < NR_TOP_LOOPS; loop++) {
		u64 time_uc, time_wb;

		set_pat(0);
		time_uc = time_memory_accesses();

		set_pat(0x0606060606060606ULL);
		time_wb = time_memory_accesses();

		if (time_uc < time_wb * 4)
			error++;

		printf("%02d uc: %10lld wb: %8lld\n", loop, time_uc, time_wb);
	}

	report("guest PAT", !error);

	return report_summary();
}