Re: [PATCH v4] VMX: use a single, global APIC access page

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Apr 12, 2021 at 12:40:48PM +0200, Jan Beulich wrote:
> The address of this page is used by the CPU only to recognize when to
> access the virtual APIC page instead. No accesses would ever go to this
> page. It only needs to be present in the (CPU) page tables so that
> address translation will produce its address as result for respective
> accesses.
> 
> By making this page global, we also eliminate the need to refcount it,
> or to assign it to any domain in the first place.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> ---
> v4: Set PGC_extra on the page. Make shadow mode work.
> v3: Split p2m insertion change to a separate patch.
> v2: Avoid insertion when !has_vlapic(). Split off change to
>     p2m_get_iommu_flags().
> ---
> I did further consider not allocating any real page at all, but just
> using the address of some unpopulated space (which would require
> announcing this page as reserved to Dom0, so it wouldn't put any PCI
> MMIO BARs there). But I thought this would be too controversial, because
> of the possible risks associated with this.

Really seems more trouble than reward. Also there are systems with
MMIO regions in holes on the memory map, like the issue I had with the
Intel pinctrl stuff that had an MMIO region in a hole on the memory
map [0], so I'm not sure Xen would be in a position to select a
suitable unpopulated page anyway.

[0] https://lore.kernel.org/xen-devel/YFx80wYt%2FKcHanC7@smile.fi.intel.com/

> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -66,8 +66,7 @@ boolean_param("force-ept", opt_force_ept
>  static void vmx_ctxt_switch_from(struct vcpu *v);
>  static void vmx_ctxt_switch_to(struct vcpu *v);
>  
> -static int  vmx_alloc_vlapic_mapping(struct domain *d);
> -static void vmx_free_vlapic_mapping(struct domain *d);
> +static int alloc_vlapic_mapping(void);
>  static void vmx_install_vlapic_mapping(struct vcpu *v);
>  static void vmx_update_guest_cr(struct vcpu *v, unsigned int cr,
>                                  unsigned int flags);
> @@ -78,6 +77,8 @@ static int vmx_msr_read_intercept(unsign
>  static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content);
>  static void vmx_invlpg(struct vcpu *v, unsigned long linear);
>  
> +static mfn_t __read_mostly apic_access_mfn;
> +
>  /* Values for domain's ->arch.hvm_domain.pi_ops.flags. */
>  #define PI_CSW_FROM (1u << 0)
>  #define PI_CSW_TO   (1u << 1)
> @@ -401,7 +402,6 @@ static int vmx_domain_initialise(struct
>          .to   = vmx_ctxt_switch_to,
>          .tail = vmx_do_resume,
>      };
> -    int rc;
>  
>      d->arch.ctxt_switch = &csw;
>  
> @@ -411,28 +411,22 @@ static int vmx_domain_initialise(struct
>       */
>      d->arch.hvm.vmx.exec_sp = is_hardware_domain(d) || opt_ept_exec_sp;
>  
> -    if ( !has_vlapic(d) )
> -        return 0;
> -
> -    if ( (rc = vmx_alloc_vlapic_mapping(d)) != 0 )
> -        return rc;
> -
>      return 0;
>  }
>  
> -static void vmx_domain_relinquish_resources(struct domain *d)
> +static void domain_creation_finished(struct domain *d)
>  {
> -    if ( !has_vlapic(d) )
> +    gfn_t gfn = gaddr_to_gfn(APIC_DEFAULT_PHYS_BASE);

Worth making it const static?

> +    uint8_t ipat;
> +
> +    if ( !has_vlapic(d) || mfn_eq(apic_access_mfn, _mfn(0)) )

It would be better to use INVALID_MFN here, and init apic_access_mfn
to that value.

>          return;
>  
> -    vmx_free_vlapic_mapping(d);
> -}
> +    ASSERT(epte_get_entry_emt(d, gfn_x(gfn), apic_access_mfn, 0, &ipat,
> +                              true) == MTRR_TYPE_WRBACK);
> +    ASSERT(ipat);
>  
> -static void domain_creation_finished(struct domain *d)
> -{
> -    if ( has_vlapic(d) && !mfn_eq(d->arch.hvm.vmx.apic_access_mfn, _mfn(0)) &&
> -         set_mmio_p2m_entry(d, gaddr_to_gfn(APIC_DEFAULT_PHYS_BASE),
> -                            d->arch.hvm.vmx.apic_access_mfn, PAGE_ORDER_4K) )
> +    if ( set_mmio_p2m_entry(d, gfn, apic_access_mfn, PAGE_ORDER_4K) )
>          domain_crash(d);
>  }
>  
> @@ -2415,7 +2409,6 @@ static struct hvm_function_table __initd
>      .cpu_up_prepare       = vmx_cpu_up_prepare,
>      .cpu_dead             = vmx_cpu_dead,
>      .domain_initialise    = vmx_domain_initialise,
> -    .domain_relinquish_resources = vmx_domain_relinquish_resources,
>      .domain_creation_finished = domain_creation_finished,
>      .vcpu_initialise      = vmx_vcpu_initialise,
>      .vcpu_destroy         = vmx_vcpu_destroy,
> @@ -2662,7 +2655,7 @@ const struct hvm_function_table * __init
>  {
>      set_in_cr4(X86_CR4_VMXE);
>  
> -    if ( vmx_vmcs_init() )
> +    if ( vmx_vmcs_init() || alloc_vlapic_mapping() )
>      {
>          printk("VMX: failed to initialise.\n");
>          return NULL;
> @@ -3224,7 +3217,7 @@ gp_fault:
>      return X86EMUL_EXCEPTION;
>  }
>  
> -static int vmx_alloc_vlapic_mapping(struct domain *d)
> +static int __init alloc_vlapic_mapping(void)
>  {
>      struct page_info *pg;
>      mfn_t mfn;
> @@ -3232,52 +3225,31 @@ static int vmx_alloc_vlapic_mapping(stru
>      if ( !cpu_has_vmx_virtualize_apic_accesses )
>          return 0;
>  
> -    pg = alloc_domheap_page(d, MEMF_no_refcount);
> +    pg = alloc_domheap_page(NULL, 0);
>      if ( !pg )
>          return -ENOMEM;
>  
> -    if ( !get_page_and_type(pg, d, PGT_writable_page) )
> -    {
> -        /*
> -         * The domain can't possibly know about this page yet, so failure
> -         * here is a clear indication of something fishy going on.
> -         */
> -        domain_crash(d);
> -        return -ENODATA;
> -    }
> +    /* Arrange for epte_get_entry_emt() to recognize this page as "special". */
> +    pg->count_info |= PGC_extra;
>  
>      mfn = page_to_mfn(pg);
>      clear_domain_page(mfn);
> -    d->arch.hvm.vmx.apic_access_mfn = mfn;
> +    apic_access_mfn = mfn;
>  
>      return 0;
>  }
>  
> -static void vmx_free_vlapic_mapping(struct domain *d)
> -{
> -    mfn_t mfn = d->arch.hvm.vmx.apic_access_mfn;
> -
> -    d->arch.hvm.vmx.apic_access_mfn = _mfn(0);
> -    if ( !mfn_eq(mfn, _mfn(0)) )
> -    {
> -        struct page_info *pg = mfn_to_page(mfn);
> -
> -        put_page_alloc_ref(pg);
> -        put_page_and_type(pg);
> -    }
> -}
> -
>  static void vmx_install_vlapic_mapping(struct vcpu *v)
>  {
>      paddr_t virt_page_ma, apic_page_ma;
>  
> -    if ( mfn_eq(v->domain->arch.hvm.vmx.apic_access_mfn, _mfn(0)) )
> +    if ( !has_vlapic(v->domain) || mfn_eq(apic_access_mfn, _mfn(0)) )
>          return;
>  
>      ASSERT(cpu_has_vmx_virtualize_apic_accesses);
>  
>      virt_page_ma = page_to_maddr(vcpu_vlapic(v)->regs_page);
> -    apic_page_ma = mfn_to_maddr(v->domain->arch.hvm.vmx.apic_access_mfn);
> +    apic_page_ma = mfn_to_maddr(apic_access_mfn);
>  
>      vmx_vmcs_enter(v);
>      __vmwrite(VIRTUAL_APIC_PAGE_ADDR, virt_page_ma);
> --- a/xen/arch/x86/mm/shadow/set.c
> +++ b/xen/arch/x86/mm/shadow/set.c
> @@ -94,6 +94,22 @@ shadow_get_page_from_l1e(shadow_l1e_t sl
>      ASSERT(!sh_l1e_is_magic(sl1e));
>      ASSERT(shadow_mode_refcounts(d));
>  
> +    /*
> +     * VMX'es APIC access MFN is just a surrogate page.  It doesn't actually
> +     * get accessed, and hence there's no need to refcount it (and refcounting
> +     * would fail, due to the page having no owner).
> +     */
> +    if ( mfn_valid(mfn = shadow_l1e_get_mfn(sl1e)) )

I find this assignment inside the parameter list quite ugly, I would
rather split it on it's own line.

> +    {
> +        const struct page_info *pg = mfn_to_page(mfn);
> +
> +        if ( !page_get_owner(pg) && (pg->count_info & PGC_extra) )
> +        {
> +            ASSERT(type == p2m_mmio_direct);
> +            return 0;

Are there any other pages that could pass this check? I don't think
so, but wanted to assert.

Thanks, Roger.