Re: KVM's support for non default APIC base

[Maxim Levitsky]

On Thu, 2021-07-22 at 12:12 +0300, Maxim Levitsky wrote:
> On Mon, 2021-07-19 at 18:49 +0000, Sean Christopherson wrote:
> > On Sun, Jul 18, 2021, Maxim Levitsky wrote:
> > > I am more inclined to fix this by just tracking if we hold the srcu
> > > lock on each VCPU manually, just as we track the srcu index anyway,
> > > and then kvm_request_apicv_update can use this to drop the srcu
> > > lock when needed.
> > 
> > The entire approach of dynamically adding/removing the memslot seems doomed to
> > failure, and is likely responsible for the performance issues with AVIC, e.g. a
> > single vCPU temporarily inhibiting AVIC will zap all SPTEs _twice_; on disable
> > and again on re-enable.
> > 
> > Rather than pile on more gunk, what about special casing the APIC access page
> > memslot in try_async_pf()?  E.g. zap the GFN in avic_update_access_page() when
> > disabling (and bounce through kvm_{inc,dec}_notifier_count()), and have the page
> > fault path skip directly to MMIO emulation without caching the MMIO info.  It'd
> > also give us a good excuse to rename try_async_pf() :-)
> > 
> > If lack of MMIO caching is a performance problem, an alternative solution would
> > be to allow caching but add a helper to zap the MMIO SPTE and request all vCPUs to
> > clear their cache.
> > 
> > It's all a bit gross, especially hijacking the mmu_notifier path, but IMO it'd be
> > less awful than the current memslot+SRCU mess
> 
> Hi Sean, Paolo and everyone else:
> 
> I am exploring the approach that you proposed and I noticed that we have very inconsistient
> code that handles the APIC base relocation for in-kernel local apic.
> I do know that APIC base relocation is not supported, and I don't have anything against
> this as long as VMs don't use that feature, but I do want this to be consistent.
> 
> I did a study of the code that is involved in this mess and I would like to hear your opinion:
> 
> There are basically 3 modes of operation of in kernel local apic:
> 
> Regular unaccelerated local apic:
> 
> -> APIC MMIO base address is stored at 'apic->base_address', and updated in 
>    kvm_lapic_set_base which is called from  msr write of 'MSR_IA32_APICBASE'
>    (both by SVM and VMX).
>    The value is even migrated.
> 
> -> APIC mmio read/write is done from MMU, when we access MMIO page:
> 	vcpu_mmio_write always calls apic_mmio_write which checks if the write is in 
> 	apic->base_address page and if so forwards the write local apic with offset
> 	in this page.
> 
> -> APIC MMIO area has to be MMIO for 'apic_mmio_write' to be called,
>    thus must contain no guest memslots.
>    If the guest relocates the APIC base somewhere where we have a memslot, 
>    memslot will take priority, while on real hardware, LAPIC is likely to
>    take priority.
> 
> APICv:
> 
> -> The default apic MMIO base (0xfee00000) is covered by a dummy page which is
>    allocated from qemu's process  using __x86_set_memory_region.
>    
>    This is done once in alloc_apic_access_page which is called on vcpu creation,
>    (but only once when the memslot is not yet enabled)
> 
> -> to avoid pinning this page into qemu's memory, reference to it
>    is dropped in alloc_apic_access_page.
>    (see commit c24ae0dcd3e8695efa43e71704d1fc4bc7e29e9b)
> 
> -> kvm_arch_mmu_notifier_invalidate_range -> checks if we invalidate GPA 0xfee00000 
>    and if so, raises KVM_REQ_APIC_PAGE_RELOAD request.
> 
> -> kvm_vcpu_reload_apic_access_page handles the KVM_REQ_APIC_PAGE_RELOAD request by calling
>    kvm_x86_ops.set_apic_access_page_addr which is only implemented on VMX
>    (vmx_set_apic_access_page_addr)
> 
> -> vmx_set_apic_access_page_addr does gfn_to_page on 0xfee00000 GPA,
>    and if the result is valid, writes the physical address of this page to APIC_ACCESS_ADDR vmcs field.
> 
>    (This is a major difference from the AVIC - AVIC's avic_vapic_bar is *GPA*, while APIC_ACCESS_ADDR
>    is host physical address which the hypervisor is supposed to map at APIC MMIO GPA using EPT)
> 
>    Note that if we have an error here, we might end with invalid APIC_ACCESS_ADDR field.
> 
> -> writes to the  HPA of that special page (which has GPA of 0xfee00000, and mapped via EPT) go to
>    APICv or cause special VM exits: (EXIT_REASON_APIC_ACCESS, EXIT_REASON_APIC_WRITE)
> 
>    * EXIT_REASON_APIC_ACCESS (which is used for older limited 'flexpriotiy' mode which only emulates TPR practically) 
>      actually emulates the instruction to know the written value,
>      but we have a special case in vcpu_is_mmio_gpa which makes the emulation treat the access to the default
>      apic base as MMIO.
>    
>    * EXIT_REASON_APIC_WRITE is a trap VMexit which comes with full APICv, and since it also has offset
>      qualification and the value is already in the apic page, this info is just passed to kvm_lapic_reg_write
> 
> 
> -> If APIC base is relocated, the APICv isn't aware of it, and the writes to new APIC base,
>    (assuming that we have no memslots covering it) will go through standard APIC MMIO emulation,
>    and *might* create a mess.
> 
> AVIC:
> 
> -> The default apic MMIO base (0xfee00000) 
>    is also covered by a dummy page which is allocated from qemu's process using __x86_set_memory_region 
>    in avic_update_access_page which is called also on vcpu creation (also only once),
>    and from SVM's dynamic AVIC inhibition.
> 
> -> The reference to this page is not dropped thus there is no KVM_REQ_APIC_PAGE_RELOAD handler.
>    I think we should do the same we do for APICv here?
> 
> -> avic_vapic_bar is GPA and thus contains 0xfee00000 but writes to MSR_IA32_APICBASE do update it
>    (avic_update_vapic_bar which is called from MSR_IA32_APICBASE write in SVM code)
> 
>    thus if the guest relocates the APIC base to a writable memory page, actually AVIC would happen to work.
>    (opposite from the stock xAPIC handlilng, which only works when apic base is in MMIO area.)
> 
> -> writes to the GPA in avic_vapic_bar are first checked in NPT (but HPA written there ignored),
>    and then either go to AVIC or cause SVM_EXIT_AVIC_UNACCELERATED_ACCESS which has offset of the write
>    in the exit_info_1
>    (there is also SVM_EXIT_AVIC_INCOMPLETE_IPI which is called on some ICR writes)
> 
> 
> As far as I know the only good reason to relocate APIC base is to access it from the real mode
> which is not something that is done these days by modern BIOSes.
> 
> I vote to make it read only (#GP on MSR_IA32_APICBASE write when non default base is set and apic enabled) 
> and remove all remains of the support for variable APIC base.
> (we already have a warning when APIC base is set to non default value)
> 
> 
> Best regards,
> 	Maxim Levitsky
> 
Ping.

Best regards,
	Maxim Levitsky