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KVM: Drop KVM's API to allow temporarily unmapping gfn=>pfn cache

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Drop kvm_gpc_unmap() as it has no users and unclear requirements.  The
API was added as part of the original gfn_to_pfn_cache support, but its
sole usage[*] was never merged.  Fold the guts of kvm_gpc_unmap() into
the deactivate path and drop the API.  Omit acquiring refresh_lock as
as concurrent calls to kvm_gpc_deactivate() are not allowed (this is
not enforced, e.g. via lockdep. due to it being called during vCPU
destruction).

If/when temporary unmapping makes a comeback, the desirable behavior is
likely to restrict temporary unmapping to vCPU-exclusive mappings and
require the vcpu->mutex be held to serialize unmap.  Use of the
refresh_lock to protect unmapping was somewhat specuatively added by
commit 93984f19e7 ("KVM: Fully serialize gfn=>pfn cache refresh via
mutex") to guard against concurrent unmaps, but the primary use case of
the temporary unmap, nested virtualization[*], doesn't actually need or
want concurrent unmaps.

[*] https://lore.kernel.org/all/20211210163625.2886-7-dwmw2@infradead.org

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
This commit is contained in:
Sean Christopherson 2022-10-13 21:12:29 +00:00 committed by David Woodhouse
parent 0318f207d1
commit 9f87791d68
2 changed files with 16 additions and 40 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
include/linux/kvm_host.h
View File

@ -1333,18 +1333,6 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len);
*/
int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len);
/**
* kvm_gpc_unmap - temporarily unmap a gfn_to_pfn_cache.
*
* @kvm: pointer to kvm instance.
* @gpc: struct gfn_to_pfn_cache object.
*
* This unmaps the referenced page. The cache is left in the invalid state
* but at least the mapping from GPA to userspace HVA will remain cached
* and can be reused on a subsequent refresh.
*/
void kvm_gpc_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc);
/**
* kvm_gpc_deactivate - deactivate and unlink a gfn_to_pfn_cache.
*

44
virt/kvm/pfncache.c
View File

@ -333,33 +333,6 @@ out_unlock:
}
EXPORT_SYMBOL_GPL(kvm_gpc_refresh);
void kvm_gpc_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc)
{
void *old_khva;
kvm_pfn_t old_pfn;
mutex_lock(&gpc->refresh_lock);
write_lock_irq(&gpc->lock);
gpc->valid = false;
old_khva = gpc->khva - offset_in_page(gpc->khva);
old_pfn = gpc->pfn;
/*
* We can leave the GPA → uHVA map cache intact but the PFN
* lookup will need to be redone even for the same page.
*/
gpc->khva = NULL;
gpc->pfn = KVM_PFN_ERR_FAULT;
write_unlock_irq(&gpc->lock);
mutex_unlock(&gpc->refresh_lock);
gpc_unmap_khva(old_pfn, old_khva);
}
EXPORT_SYMBOL_GPL(kvm_gpc_unmap);
void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm,
struct kvm_vcpu *vcpu, enum pfn_cache_usage usage)
{
@ -405,6 +378,8 @@ EXPORT_SYMBOL_GPL(kvm_gpc_activate);
void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
{
struct kvm *kvm = gpc->kvm;
kvm_pfn_t old_pfn;
void *old_khva;
if (gpc->active) {
/*
@ -414,13 +389,26 @@ void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
*/
write_lock_irq(&gpc->lock);
gpc->active = false;
gpc->valid = false;
/*
* Leave the GPA => uHVA cache intact, it's protected by the
* memslot generation. The PFN lookup needs to be redone every
* time as mmu_notifier protection is lost when the cache is
* removed from the VM's gpc_list.
*/
old_khva = gpc->khva - offset_in_page(gpc->khva);
gpc->khva = NULL;
old_pfn = gpc->pfn;
gpc->pfn = KVM_PFN_ERR_FAULT;
write_unlock_irq(&gpc->lock);
spin_lock(&kvm->gpc_lock);
list_del(&gpc->list);
spin_unlock(&kvm->gpc_lock);
kvm_gpc_unmap(kvm, gpc);
gpc_unmap_khva(old_pfn, old_khva);
}
}
EXPORT_SYMBOL_GPL(kvm_gpc_deactivate);