Re: OVMF very slow on AMD

[Anthony PERARD <anthony.perard@citrix.com>]

On Fri, Jul 15, 2016 at 09:48:31AM -0400, Konrad Rzeszutek Wilk wrote:
> On Thu, Jul 14, 2016 at 04:53:07PM +0100, Anthony PERARD wrote:
> > So, this loop takes about 1 minute on my AMD machine (AMD Opteron(tm)
> > Processor 4284), and less that 1 second on an Intel machine.
> > If I compile OVMF as a 32bit binary, the loop is faster, but still takes
> > about 30s on AMD. (that's true for both OvmfIa32 and OvmfIa32X64 which
> > is 32bit bootstrap, but can start 64bit OS.)
> > Another thing, I tried the same binary (64bit) with KVM, and OVMF seems
> > fast.
> > 
> > 
> > So, any idee of what I could investigate?
> 
> I presume we emulating some operation on AMD but not on Intel.
> 
> However you say xentrace shows nothing - which would imply we are not
> incurring VMEXITs to deal with this. Hmm.. Could it be what we
> expose to the guest (the CPUID flags?) Somehow we are missing one on AMD
> and it takes a slower route?

Since the same binary runs much faster in a KVM guest, I have compared
the procinfo of the guest between KVM and Xen (via /proc/cpuinfo), and
Xen have more flags:

$ dwdiff procinfo_guest_ovmf_kvm procinfo_guest_ovmf_xen
processor	: 0
vendor_id	: AuthenticAMD
cpu family	: [-6-] {+21+}
model		: [-6-] {+1+}
model name	: [-QEMU Virtual CPU version 2.5+-] {+AMD Opteron(tm) Processor 4284+}
stepping	: [-3-] {+2+}
microcode	: [-0x1000065-] {+0x600063d+}
cpu MHz		: [-3000.034-] {+3000.112+}
cache size	: [-512-] {+2048+} KB
{+physical id	: 0
siblings	: 1
core id		: 0
cpu cores	: 1
apicid		: 0
initial apicid	: 0+}
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu {+vme+} de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 {+ht+} syscall nx {+mmxext fxsr_opt pdpe1gb rdtscp+} lm {+rep_good+} nopl {+extd_apicid+} pni {+pclmulqdq ssse3+} cx16 {+sse4_1 sse4_2+} x2apic {+popcnt aes xsave avx+} hypervisor lahf_lm {+cr8_legacy abm sse4a misalignsse 3dnowprefetch ibs xop lwp fma4 arat+}
bogomips	: [-6002.07-] {+6002.23+}
TLB size	: [-1024-] {+1536+} 4K pages
clflush size	: 64
cache_alignment	: 64
address sizes	: [-40-] {+48+} bits physical, 48 bits virtual
power management:


-- 
Anthony PERARD

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