Re: [PATCH RESEND v4 7/9] KVM: VMX: Handle SPP induced vmexit and page fault
On Thu, Aug 22, 2019 at 06:38:41PM +0200, Paolo Bonzini wrote: > On 22/08/19 15:17, Yang Weijiang wrote: > > On Tue, Aug 20, 2019 at 09:44:35PM +0800, Yang Weijiang wrote: > >> On Mon, Aug 19, 2019 at 05:04:23PM +0200, Paolo Bonzini wrote: > >>> fast_page_fault should never trigger an SPP userspace exit on its own, > >>> all the SPP handling should go through handle_spp. > > Hi, Paolo, > > According to the latest SDM(28.2.4), handle_spp only handles SPPT miss and > > SPPT > > misconfig(exit_reason==66), subpage write access violation causes EPT > > violation, > > so have to deal with the two cases into handlers. > > Ok, so this part has to remain, though you do have to save/restore > PT_SPP_MASK according to the rest of the email. > > Paolo > Got it, thanks! > >>> So I think that when KVM wants to write-protect the whole page > >>> (wrprot_ad_disabled_spte) it must also clear PT_SPP_MASK; for example it > >>> could save it in bit 53 (PT64_SECOND_AVAIL_BITS_SHIFT + 1). If the > >>> saved bit is set, fast_page_fault must then set PT_SPP_MASK instead of > >>> PT_WRITABLE_MASK.
Re: [PATCH RESEND v4 7/9] KVM: VMX: Handle SPP induced vmexit and page fault
On 22/08/19 15:17, Yang Weijiang wrote: > On Tue, Aug 20, 2019 at 09:44:35PM +0800, Yang Weijiang wrote: >> On Mon, Aug 19, 2019 at 05:04:23PM +0200, Paolo Bonzini wrote: >>> fast_page_fault should never trigger an SPP userspace exit on its own, >>> all the SPP handling should go through handle_spp. > Hi, Paolo, > According to the latest SDM(28.2.4), handle_spp only handles SPPT miss and > SPPT > misconfig(exit_reason==66), subpage write access violation causes EPT > violation, > so have to deal with the two cases into handlers. Ok, so this part has to remain, though you do have to save/restore PT_SPP_MASK according to the rest of the email. Paolo >>> So I think that when KVM wants to write-protect the whole page >>> (wrprot_ad_disabled_spte) it must also clear PT_SPP_MASK; for example it >>> could save it in bit 53 (PT64_SECOND_AVAIL_BITS_SHIFT + 1). If the >>> saved bit is set, fast_page_fault must then set PT_SPP_MASK instead of >>> PT_WRITABLE_MASK.
Re: [PATCH RESEND v4 7/9] KVM: VMX: Handle SPP induced vmexit and page fault
On Tue, Aug 20, 2019 at 09:44:35PM +0800, Yang Weijiang wrote:
> On Mon, Aug 19, 2019 at 05:04:23PM +0200, Paolo Bonzini wrote:
> > On 19/08/19 16:43, Paolo Bonzini wrote:
> > >> +/*
> > >> + * Record write protect fault caused by
> > >> + * Sub-page Protection, let VMI decide
> > >> + * the next step.
> > >> + */
> > >> +if (spte & PT_SPP_MASK) {
> > > Should this be "if (spte & PT_WRITABLE_MASK)" instead? That is, if the
> > > page is already writable, the fault must be an SPP fault.
> >
> > Hmm, no I forgot how SPP works; still, this is *not* correct. For
> > example, if SPP marks part of a page as read-write, but KVM wants to
> > write-protect the whole page for access or dirty tracking, that should
> > not cause an SPP exit.
> >
> > So I think that when KVM wants to write-protect the whole page
> > (wrprot_ad_disabled_spte) it must also clear PT_SPP_MASK; for example it
> > could save it in bit 53 (PT64_SECOND_AVAIL_BITS_SHIFT + 1). If the
> > saved bit is set, fast_page_fault must then set PT_SPP_MASK instead of
> > PT_WRITABLE_MASK.
> Sure, will change the processing flow.
>
> > On re-entry this will cause an SPP vmexit;
> > fast_page_fault should never trigger an SPP userspace exit on its own,
> > all the SPP handling should go through handle_spp.
Hi, Paolo,
According to the latest SDM(28.2.4), handle_spp only handles SPPT miss and SPPT
misconfig(exit_reason==66), subpage write access violation causes EPT
violation,
so have to deal with the two cases into handlers.
> > Paolo
Re: [PATCH RESEND v4 7/9] KVM: VMX: Handle SPP induced vmexit and page fault
On Mon, Aug 19, 2019 at 05:04:23PM +0200, Paolo Bonzini wrote:
> On 19/08/19 16:43, Paolo Bonzini wrote:
> >> + /*
> >> + * Record write protect fault caused by
> >> + * Sub-page Protection, let VMI decide
> >> + * the next step.
> >> + */
> >> + if (spte & PT_SPP_MASK) {
> > Should this be "if (spte & PT_WRITABLE_MASK)" instead? That is, if the
> > page is already writable, the fault must be an SPP fault.
>
> Hmm, no I forgot how SPP works; still, this is *not* correct. For
> example, if SPP marks part of a page as read-write, but KVM wants to
> write-protect the whole page for access or dirty tracking, that should
> not cause an SPP exit.
>
> So I think that when KVM wants to write-protect the whole page
> (wrprot_ad_disabled_spte) it must also clear PT_SPP_MASK; for example it
> could save it in bit 53 (PT64_SECOND_AVAIL_BITS_SHIFT + 1). If the
> saved bit is set, fast_page_fault must then set PT_SPP_MASK instead of
> PT_WRITABLE_MASK.
Sure, will change the processing flow.
> On re-entry this will cause an SPP vmexit;
> fast_page_fault should never trigger an SPP userspace exit on its own,
> all the SPP handling should go through handle_spp.
>
> Paolo
Re: [PATCH RESEND v4 7/9] KVM: VMX: Handle SPP induced vmexit and page fault
On 19/08/19 16:43, Paolo Bonzini wrote:
>> +/*
>> + * Record write protect fault caused by
>> + * Sub-page Protection, let VMI decide
>> + * the next step.
>> + */
>> +if (spte & PT_SPP_MASK) {
> Should this be "if (spte & PT_WRITABLE_MASK)" instead? That is, if the
> page is already writable, the fault must be an SPP fault.
Hmm, no I forgot how SPP works; still, this is *not* correct. For
example, if SPP marks part of a page as read-write, but KVM wants to
write-protect the whole page for access or dirty tracking, that should
not cause an SPP exit.
So I think that when KVM wants to write-protect the whole page
(wrprot_ad_disabled_spte) it must also clear PT_SPP_MASK; for example it
could save it in bit 53 (PT64_SECOND_AVAIL_BITS_SHIFT + 1). If the
saved bit is set, fast_page_fault must then set PT_SPP_MASK instead of
PT_WRITABLE_MASK. On re-entry this will cause an SPP vmexit;
fast_page_fault should never trigger an SPP userspace exit on its own,
all the SPP handling should go through handle_spp.
Paolo
Re: [PATCH RESEND v4 7/9] KVM: VMX: Handle SPP induced vmexit and page fault
On 14/08/19 09:04, Yang Weijiang wrote:
> + /*
> + * Record write protect fault caused by
> + * Sub-page Protection, let VMI decide
> + * the next step.
> + */
> + if (spte & PT_SPP_MASK) {
Should this be "if (spte & PT_WRITABLE_MASK)" instead? That is, if the
page is already writable, the fault must be an SPP fault.
Paolo
> + fault_handled = true;
> + vcpu->run->exit_reason = KVM_EXIT_SPP;
> + vcpu->run->spp.addr = gva;
> + kvm_skip_emulated_instruction(vcpu);
> + break;
> + }
> +
> new_spte |= PT_WRITABLE_MASK;
