I believe that ESXi reads GUEST_CS_AR_BYTES on every VM-exit to
determine code size.
On Wed, Oct 17, 2018 at 10:02 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> On 17/10/2018 16:47, Vitaly Kuznetsov wrote:
>>>> + if (!hv_evmcs || !(hv_evmcs->hv_clean_fields &
>>>> + HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP2)) {
>>>> + vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector);
>>>> + vmcs_write32(GUEST_CS_LIMIT, vmcs12->guest_cs_limit);
>>>> + vmcs_write32(GUEST_CS_AR_BYTES, vmcs12->guest_cs_ar_bytes);
>>>> + vmcs_writel(GUEST_ES_BASE, vmcs12->guest_es_base);
>>>> + vmcs_writel(GUEST_CS_BASE, vmcs12->guest_cs_base);
>>>> + }
>>> For what it's worth, I suspect that these can be moved to
>>> prepare_vmcs02_full. The initial implementation of shadow VMCS did not
>>> expose "unrestricted guest" to the L1 hypervisor, and emulation does a
>>> lot of accesses to CS (of course). Not sure how ES base ended up in
>>> there and not DS base, though...
>> I tried unshadowing all these fields and at least Hyper-V on KVM
>> (without using eVMCS of course) experiences a 1200-1300 cpu cycles
>> regression during tight cpuid loop test. I checked and this happens
>> because it likes vmreading GUEST_CS_AR_BYTES a lot.
>
> Go figure. :) Liran, do you happen to know if ESX does something
> similar with CS descriptor cache fields?
>
> Paolo
