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KVM: x86/xen: Maintain valid mapping of Xen shared_info page

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Use the newly reinstated gfn_to_pfn_cache to maintain a kernel mapping
of the Xen shared_info page so that it can be accessed in atomic context.

Note that we do not participate in dirty tracking for the shared info
page and we do not explicitly mark it dirty every single tim we deliver
an event channel interrupts. We wouldn't want to do that even if we *did*
have a valid vCPU context with which to do so.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20211210163625.2886-4-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
David Woodhouse 2021-12-10 16:36:22 +00:00 committed by Paolo Bonzini
parent 982ed0de47
commit 1cfc9c4b9d
3 changed files with 27 additions and 12 deletions
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12
Documentation/virt/kvm/api.rst
View File

@ -371,6 +371,9 @@ The bits in the dirty bitmap are cleared before the ioctl returns, unless
KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 is enabled. For more information,
see the description of the capability.
Note that the Xen shared info page, if configured, shall always be assumed
to be dirty. KVM will not explicitly mark it such.
4.9 KVM_SET_MEMORY_ALIAS
------------------------
@ -5134,6 +5137,15 @@ KVM_XEN_ATTR_TYPE_SHARED_INFO
not aware of the Xen CPU id which is used as the index into the
vcpu_info[] array, so cannot know the correct default location.
Note that the shared info page may be constantly written to by KVM;
it contains the event channel bitmap used to deliver interrupts to
a Xen guest, amongst other things. It is exempt from dirty tracking
mechanisms — KVM will not explicitly mark the page as dirty each
time an event channel interrupt is delivered to the guest! Thus,
userspace should always assume that the designated GFN is dirty if
any vCPU has been running or any event channel interrupts can be
routed to the guest.
KVM_XEN_ATTR_TYPE_UPCALL_VECTOR
Sets the exception vector used to deliver Xen event channel upcalls.

2
arch/x86/include/asm/kvm_host.h
View File

@ -1020,7 +1020,7 @@ struct msr_bitmap_range {
struct kvm_xen {
bool long_mode;
u8 upcall_vector;
gfn_t shinfo_gfn;
struct gfn_to_pfn_cache shinfo_cache;
};
enum kvm_irqchip_mode {

25
arch/x86/kvm/xen.c
View File

@ -23,16 +23,21 @@ DEFINE_STATIC_KEY_DEFERRED_FALSE(kvm_xen_enabled, HZ);
static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn)
{
struct gfn_to_pfn_cache *gpc = &kvm->arch.xen.shinfo_cache;
gpa_t gpa = gfn_to_gpa(gfn);
int wc_ofs, sec_hi_ofs;
int ret = 0;
int idx = srcu_read_lock(&kvm->srcu);
if (kvm_is_error_hva(gfn_to_hva(kvm, gfn))) {
ret = -EFAULT;
if (gfn == GPA_INVALID) {
kvm_gfn_to_pfn_cache_destroy(kvm, gpc);
goto out;
}
kvm->arch.xen.shinfo_gfn = gfn;
ret = kvm_gfn_to_pfn_cache_init(kvm, gpc, NULL, false, true, gpa,
PAGE_SIZE, false);
if (ret)
goto out;
/* Paranoia checks on the 32-bit struct layout */
BUILD_BUG_ON(offsetof(struct compat_shared_info, wc) != 0x900);
@ -260,15 +265,9 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
break;
case KVM_XEN_ATTR_TYPE_SHARED_INFO:
if (data->u.shared_info.gfn == GPA_INVALID) {
kvm->arch.xen.shinfo_gfn = GPA_INVALID;
r = 0;
break;
}
r = kvm_xen_shared_info_init(kvm, data->u.shared_info.gfn);
break;
case KVM_XEN_ATTR_TYPE_UPCALL_VECTOR:
if (data->u.vector && data->u.vector < 0x10)
r = -EINVAL;
@ -299,7 +298,10 @@ int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
break;
case KVM_XEN_ATTR_TYPE_SHARED_INFO:
data->u.shared_info.gfn = kvm->arch.xen.shinfo_gfn;
if (kvm->arch.xen.shinfo_cache.active)
data->u.shared_info.gfn = gpa_to_gfn(kvm->arch.xen.shinfo_cache.gpa);
else
data->u.shared_info.gfn = GPA_INVALID;
r = 0;
break;
@ -661,11 +663,12 @@ int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc)
void kvm_xen_init_vm(struct kvm *kvm)
{
kvm->arch.xen.shinfo_gfn = GPA_INVALID;
}
void kvm_xen_destroy_vm(struct kvm *kvm)
{
kvm_gfn_to_pfn_cache_destroy(kvm, &kvm->arch.xen.shinfo_cache);
if (kvm->arch.xen_hvm_config.msr)
static_branch_slow_dec_deferred(&kvm_xen_enabled);
}