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Re: [PATCH 06/22] x86: map/unmap pages in restore_all_guests

To: Jan Beulich <jbeulich@xxxxxxxx>
From: Julien Grall <julien@xxxxxxx>
Date: Thu, 22 Jun 2023 11:44:12 +0100
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Julien Grall <jgrall@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
Delivery-date: Thu, 22 Jun 2023 10:44:31 +0000
List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

Hi Jan,

Sorry for the late reply.

On 13/01/2023 09:22, Jan Beulich wrote:

On 13.01.2023 00:20, Julien Grall wrote:

On 04/01/2023 10:27, Jan Beulich wrote:

On 23.12.2022 13:22, Julien Grall wrote:

On 22/12/2022 11:12, Jan Beulich wrote:

On 16.12.2022 12:48, Julien Grall wrote:

--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -165,7 +165,24 @@ restore_all_guest:
            and   %rsi, %rdi
            and   %r9, %rsi
            add   %rcx, %rdi
-        add   %rcx, %rsi
+
+         /*
+          * Without a direct map, we have to map first before copying. We only
+          * need to map the guest root table but not the per-CPU root_pgt,
+          * because the latter is still a xenheap page.
+          */
+        pushq %r9
+        pushq %rdx
+        pushq %rax
+        pushq %rdi
+        mov   %rsi, %rdi
+        shr   $PAGE_SHIFT, %rdi
+        callq map_domain_page
+        mov   %rax, %rsi
+        popq  %rdi
+        /* Stash the pointer for unmapping later. */
+        pushq %rax
+
            mov   $ROOT_PAGETABLE_FIRST_XEN_SLOT, %ecx
            mov   root_table_offset(SH_LINEAR_PT_VIRT_START)*8(%rsi), %r8
            mov   %r8, root_table_offset(SH_LINEAR_PT_VIRT_START)*8(%rdi)
@@ -177,6 +194,14 @@ restore_all_guest:
            sub   $(ROOT_PAGETABLE_FIRST_XEN_SLOT - \
                    ROOT_PAGETABLE_LAST_XEN_SLOT - 1) * 8, %rdi
            rep movsq
+
+        /* Unmap the page. */
+        popq  %rdi
+        callq unmap_domain_page
+        popq  %rax
+        popq  %rdx
+        popq  %r9


While the PUSH/POP are part of what I dislike here, I think this wants
doing differently: Establish a mapping when putting in place a new guest
page table, and use the pointer here. This could be a new per-domain
mapping, to limit its visibility.


I have looked at a per-domain approach and this looks way more complex
than the few concise lines here (not mentioning the extra amount of
memory).


Yes, I do understand that would be a more intrusive change.


I could be persuaded to look at a more intrusive change if there are a
good reason to do it. To me, at the moment, it mostly seem a matter of
taste.

So what would we gain from a perdomain mapping?


Rather than mapping/unmapping once per hypervisor entry/exit, we'd
map just once per context switch. Plus we'd save ugly/fragile assembly
code (apart from the push/pop I also dislike C functions being called
from assembly which aren't really meant to be called this way: While
these two may indeed be unlikely to ever change, any such change comes
with the risk of the assembly callers being missed - the compiler
won't tell you that e.g. argument types/count don't match parameters
anymore).

I think I have managed to write what you suggested. I would like toshare to get early feedback before resending the series.

There are also a couple of TODOs (XXX) in place where I am not sure ifthis is correct.

diff --git a/xen/arch/x86/include/asm/config.hb/xen/arch/x86/include/asm/config.h

index fbc4bb3416bd..320ddb9e1e77 100644
--- a/xen/arch/x86/include/asm/config.h
+++ b/xen/arch/x86/include/asm/config.h
@@ -202,7 +202,7 @@ extern unsigned char boot_edid_info[128];
 /* Slot 260: per-domain mappings (including map cache). */
 #define PERDOMAIN_VIRT_START    (PML4_ADDR(260))

#define PERDOMAIN_SLOT_MBYTES (PML4_ENTRY_BYTES >> (20 +PAGETABLE_ORDER))

-#define PERDOMAIN_SLOTS         3
+#define PERDOMAIN_SLOTS         4
 #define PERDOMAIN_VIRT_SLOT(s)  (PERDOMAIN_VIRT_START + (s) * \
                                  (PERDOMAIN_SLOT_MBYTES << 20))

/* Slot 4: mirror of per-domain mappings (for compat xlat areaaccesses). */

@@ -316,6 +316,16 @@ extern unsigned long xen_phys_start;
 #define ARG_XLAT_START(v)        \
     (ARG_XLAT_VIRT_START + ((v)->vcpu_id << ARG_XLAT_VA_SHIFT))

+/* CR3 shadow mapping area. The fourth per-domain-mapping sub-area */
+#define SHADOW_CR3_VIRT_START   PERDOMAIN_VIRT_SLOT(3)
+#define SHADOW_CR3_ENTRIES      MAX_VIRT_CPUS
+#define SHADOW_CR3_VIRT_END     (SHADOW_CR3_VIRT_START +    \
+                                 (MAX_VIRT_CPUS * PAGE_SIZE))
+
+/* The address of a particular VCPU's c3 */
+#define SHADOW_CR3_VCPU_VIRT_START(v) \
+    (SHADOW_CR3_VIRT_START + ((v)->vcpu_id * PAGE_SIZE))
+
 #define ELFSIZE 64

 #define ARCH_CRASH_SAVE_VMCOREINFO

diff --git a/xen/arch/x86/include/asm/domain.hb/xen/arch/x86/include/asm/domain.h

index c2d9fc333be5..d5989224f4a3 100644
--- a/xen/arch/x86/include/asm/domain.h
+++ b/xen/arch/x86/include/asm/domain.h
@@ -273,6 +273,7 @@ struct time_scale {
 struct pv_domain
 {
     l1_pgentry_t **gdt_ldt_l1tab;
+    l1_pgentry_t **shadow_cr3_l1tab;

     atomic_t nr_l4_pages;

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 9741d28cbc96..b64ee1ca47f6 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c

@@ -509,6 +509,13 @@ void share_xen_page_with_guest(struct page_info*page, struct domain *d,

     spin_unlock(&d->page_alloc_lock);
 }

+#define shadow_cr3_idx(v) \
+    ((v)->vcpu_id >> PAGETABLE_ORDER)
+
+#define pv_shadow_cr3_pte(v) \
+    ((v)->domain->arch.pv.shadow_cr3_l1tab[shadow_cr3_idx(v)] + \
+     ((v)->vcpu_id & (L1_PAGETABLE_ENTRIES - 1)))
+
 void make_cr3(struct vcpu *v, mfn_t mfn)
 {
     struct domain *d = v->domain;
@@ -516,6 +523,18 @@ void make_cr3(struct vcpu *v, mfn_t mfn)
     v->arch.cr3 = mfn_x(mfn) << PAGE_SHIFT;
     if ( is_pv_domain(d) && d->arch.pv.pcid )
         v->arch.cr3 |= get_pcid_bits(v, false);
+
+    /* Update the CR3 mapping */
+    if ( is_pv_domain(d) )
+    {
+        l1_pgentry_t *pte = pv_shadow_cr3_pte(v);
+
+        /* XXX Do we need to call get page first? */
+        l1e_write(pte, l1e_from_mfn(mfn, __PAGE_HYPERVISOR_RW));
+        /* XXX Can the flush be reduced to the page? */
+        /* XXX Do we always call with current? */
+        flush_tlb_local();
+    }
 }

 void write_ptbase(struct vcpu *v)
diff --git a/xen/arch/x86/pv/domain.c b/xen/arch/x86/pv/domain.c
index 5c92812dc67a..064645ccc261 100644
--- a/xen/arch/x86/pv/domain.c
+++ b/xen/arch/x86/pv/domain.c
@@ -288,6 +288,19 @@ static void pv_destroy_gdt_ldt_l1tab(struct vcpu *v)
                               1U << GDT_LDT_VCPU_SHIFT);
 }

+static int pv_create_shadow_cr3_l1tab(struct vcpu *v)
+{

+ return create_perdomain_mapping(v->domain,SHADOW_CR3_VCPU_VIRT_START(v),

+                                    1, v->domain->arch.pv.shadow_cr3_l1tab,
+                                    NULL);
+}
+
+static void pv_destroy_shadow_cr3_l1tab(struct vcpu *v)
+
+{
+    destroy_perdomain_mapping(v->domain, SHADOW_CR3_VCPU_VIRT_START(v), 1);
+}
+
 void pv_vcpu_destroy(struct vcpu *v)
 {
     if ( is_pv_32bit_vcpu(v) )
@@ -297,6 +310,7 @@ void pv_vcpu_destroy(struct vcpu *v)
     }

     pv_destroy_gdt_ldt_l1tab(v);
+    pv_destroy_shadow_cr3_l1tab(v);
     XFREE(v->arch.pv.trap_ctxt);
 }

@@ -311,6 +325,10 @@ int pv_vcpu_initialise(struct vcpu *v)
     if ( rc )
         return rc;

+    rc = pv_create_shadow_cr3_l1tab(v);
+    if ( rc )
+        goto done;
+
     BUILD_BUG_ON(X86_NR_VECTORS * sizeof(*v->arch.pv.trap_ctxt) >
                  PAGE_SIZE);

v->arch.pv.trap_ctxt = xzalloc_array(struct trap_info,X86_NR_VECTORS);

@@ -346,10 +364,12 @@ void pv_domain_destroy(struct domain *d)

     destroy_perdomain_mapping(d, GDT_LDT_VIRT_START,
                               GDT_LDT_MBYTES << (20 - PAGE_SHIFT));

+ destroy_perdomain_mapping(d, SHADOW_CR3_VIRT_START,SHADOW_CR3_ENTRIES);


     XFREE(d->arch.pv.cpuidmasks);

     FREE_XENHEAP_PAGE(d->arch.pv.gdt_ldt_l1tab);
+    FREE_XENHEAP_PAGE(d->arch.pv.shadow_cr3_l1tab);
 }

 void noreturn cf_check continue_pv_domain(void);
@@ -371,6 +391,12 @@ int pv_domain_initialise(struct domain *d)
         goto fail;
     clear_page(d->arch.pv.gdt_ldt_l1tab);

+    d->arch.pv.shadow_cr3_l1tab =
+        alloc_xenheap_pages(0, MEMF_node(domain_to_node(d)));
+    if ( !d->arch.pv.shadow_cr3_l1tab )
+        goto fail;
+    clear_page(d->arch.pv.shadow_cr3_l1tab);
+
     if ( levelling_caps & ~LCAP_faulting &&
          (d->arch.pv.cpuidmasks = xmemdup(&cpuidmask_defaults)) == NULL )
         goto fail;
@@ -381,6 +407,11 @@ int pv_domain_initialise(struct domain *d)
     if ( rc )
         goto fail;

+    rc = create_perdomain_mapping(d, SHADOW_CR3_VIRT_START,
+                                  SHADOW_CR3_ENTRIES, NULL, NULL);
+    if ( rc )
+        goto fail;
+
     d->arch.ctxt_switch = &pv_csw;

d->arch.pv.xpti = is_hardware_domain(d) ? opt_xpti_hwdom :opt_xpti_domu;diff --git a/xen/arch/x86/x86_64/asm-offsets.cb/xen/arch/x86/x86_64/asm-offsets.c

index 287dac101ad4..ed486607bf15 100644
--- a/xen/arch/x86/x86_64/asm-offsets.c
+++ b/xen/arch/x86/x86_64/asm-offsets.c
@@ -51,6 +51,7 @@ void __dummy__(void)
     OFFSET(UREGS_kernel_sizeof, struct cpu_user_regs, es);
     BLANK();

+    OFFSET(VCPU_id, struct vcpu, vcpu_id);
     OFFSET(VCPU_processor, struct vcpu, processor);
     OFFSET(VCPU_domain, struct vcpu, domain);
     OFFSET(VCPU_vcpu_info, struct vcpu, vcpu_info);
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 8b77d7113bbf..678876a32177 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -165,7 +165,16 @@ restore_all_guest:
         and   %rsi, %rdi
         and   %r9, %rsi
         add   %rcx, %rdi
+
+        /*
+         * The address in the vCPU cr3 is always mapped in the shadow
+         * cr3 virt area.
+         */
+        mov   VCPU_id(%rbx), %rsi
+        shl   $PAGE_SHIFT, %rsi
+        movabs $SHADOW_CR3_VIRT_START, %rcx
         add   %rcx, %rsi
+
         mov   $ROOT_PAGETABLE_FIRST_XEN_SLOT, %ecx
         mov   root_table_offset(SH_LINEAR_PT_VIRT_START)*8(%rsi), %r8
         mov   %r8, root_table_offset(SH_LINEAR_PT_VIRT_START)*8(%rdi)

Jan


--
Julien Grall

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