Re: [PART1 RFC v2 05/10] KVM: x86: Detect and Initialize AVIC support

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Hi

On 03/07/2016 11:41 PM, Paolo Bonzini wrote:
On 04/03/2016 21:46, Suravee Suthikulpanit wrote:
>  [....]
+/* Note: This structure is per VM */
+struct svm_vm_data {
+	atomic_t count;
+	u32 ldr_mode;
+	u32 avic_max_vcpu_id;
+	u32 avic_tag;
+
+	struct page *avic_log_ait_page;
+	struct page *avic_phy_ait_page;

You can put these directly in kvm_arch.  Do not use abbreviations:

	struct page *avic_logical_apic_id_table_page;
	struct page *avic_physical_apic_id_table_page;


Actually, the reason I would like to introduce this per-arch specific structure is because I feel that it is easier to manage these processor-specific variable/data-structure. If we add all these directly into kvm_arch, which is shared b/w SVM and VMX, it is more difficult to tell which one is used in the different code base.

[...]
+	memcpy(vapic_bkpg, svm->in_kernel_lapic_regs, PAGE_SIZE);
+	svm->vcpu.arch.apic->regs = vapic_bkpg;

Can you explain the flipping logic, and why you cannot just use the
existing apic.regs?

Please see "explanation 1" below.

[...]
+static struct svm_avic_phy_ait_entry *
+avic_get_phy_ait_entry(struct kvm_vcpu *vcpu, int index)
+{
+    [.....]
+}
+
+struct svm_avic_log_ait_entry *
+avic_get_log_ait_entry(struct kvm_vcpu *vcpu, u8 mda, bool is_flat)
+{
+	[.....]
+}

Instead of these functions, create a complete function to handle APIC_ID
and APIC_LDR writes.  Then use kmap/kunmap instead of page_address.


Ok. May I ask why we are against using page_address? I have see that used in several places in the code.

[...]
+static int avic_alloc_bk_page(struct vcpu_svm *svm, int id)
+{
+	int ret = 0, i;
+	bool realloc = false;
+	struct kvm_vcpu *vcpu;
+	struct kvm *kvm = svm->vcpu.kvm;
+	struct svm_vm_data *vm_data = kvm->arch.arch_data;
+
+	mutex_lock(&kvm->slots_lock);
+
+	/* Check if we have already allocated vAPIC backing
+	 * page for this vCPU. If not, we need to realloc
+	 * a new one and re-assign all other vCPU.
+	 */
+	if (kvm->arch.apic_access_page_done &&
+	    (id > vm_data->avic_max_vcpu_id)) {
+		kvm_for_each_vcpu(i, vcpu, kvm)
+			avic_unalloc_bk_page(vcpu);
+
+		__x86_set_memory_region(kvm, APIC_ACCESS_PAGE_PRIVATE_MEMSLOT,
+					0, 0);
+		realloc = true;
+		vm_data->avic_max_vcpu_id = 0;
+	}
+
+	/*
+	 * We are allocating vAPIC backing page
+	 * upto the max vCPU ID
+	 */
+	if (id >= vm_data->avic_max_vcpu_id) {
+		ret = __x86_set_memory_region(kvm,
+					      APIC_ACCESS_PAGE_PRIVATE_MEMSLOT,
+					      APIC_DEFAULT_PHYS_BASE,
+					      PAGE_SIZE * (id + 1));

Why is this necessary?  The APIC access page is a peculiarity of Intel
processors (and the special memslot for only needs to map 0xfee00000 to
0xfee00fff; after that there is the MSI area).


Please see "explanation 1" below.

>> [...]
+		if (ret)
+			goto out;
+
+		vm_data->avic_max_vcpu_id = id;
+	}
+
+	/* Reinit vAPIC backing page for exisinting vcpus */
+	if (realloc)
+		kvm_for_each_vcpu(i, vcpu, kvm)
+			avic_init_bk_page(vcpu);

Why is this necessary?

Explanation 1:

The current lapic regs page is allocated using get_zeroed_page(), which can be paged out. If I use these pages for AVIC backing pages, it seems to cause VM to slow down quite a bit due to a lot of page faults.

Currently, the AVIC backing pages are acquired from __x86_set_memory region() with APIC_ACCESS_PAGE_PRIVATE_MEMSLOT, which maps the pages for address 0xfee00000 and above for VM to use. I mostly grab this from the VMX implementation in alloc_apic_access_page().

However, the memslot requires specification of the size at the time when calling __x86_set_memory_region(). However, I can't seem to figure out where I can get the number of vcpus at the time when we creating VM. Therefore, I have to track the vcpu creation, and re-acquire larger memslot every time vcpu_create() is called.

I was not sure if this is the right approach, any suggestion for this part.

Thanks,
Suravee
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