Messages in this thread | | | Date | Tue, 06 Oct 2009 11:34:52 +0200 | From | Avi Kivity <> | Subject | Re: [PATCH v2 2/4] KVM: introduce "xinterface" API for external interaction with guests |
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On 10/06/2009 01:57 AM, Gregory Haskins wrote: > Avi Kivity wrote: > >> On 10/02/2009 10:19 PM, Gregory Haskins wrote: >> >>> What: xinterface is a mechanism that allows kernel modules external to >>> the kvm.ko proper to interface with a running guest. It accomplishes >>> this by creating an abstracted interface which does not expose any >>> private details of the guest or its related KVM structures, and provides >>> a mechanism to find and bind to this interface at run-time. >>> >>> >> If this is needed, it should be done as a virt_address_space to which >> kvm and other modules bind, instead of as something that kvm exports and >> other modules import. The virt_address_space can be identified by an fd >> and passed around to kvm and other modules. >> > IIUC, what you are proposing is something similar to generalizing the > vbus::memctx object. I had considered doing something like that in the > early design phase of vbus, but then decided it would be a hard-sell to > the mm crowd, and difficult to generalize. > > What do you propose as the interface to program the object? >
Something like the current kvm interfaces, de-warted. It will be a hard sell indeed, for good reasons.
>> So, under my suggestion above, you'd call >> sys_create_virt_address_space(), populate it, and pass the result to kvm >> and to foo. This allows the use of virt_address_space without kvm and >> doesn't require foo to interact with kvm. >> > The problem I see here is that the only way I can think to implement > this generally is something that looks very kvm-esque (slots-to-pages > kind of translation). Is there a way you can think of that does not > involve a kvm.ko originated vtable that is also not kvm centric? >
slots would be one implementation, if you can think of others then you'd add them.
If you can't, I think it indicates that the whole thing isn't necessary and we're better off with slots and virtual memory. The only thing missing is dma, which you don't deal with anyway.
>>> +struct kvm_xinterface_ops { >>> + unsigned long (*copy_to)(struct kvm_xinterface *intf, >>> + unsigned long gpa, const void *src, >>> + unsigned long len); >>> + unsigned long (*copy_from)(struct kvm_xinterface *intf, void *dst, >>> + unsigned long gpa, unsigned long len); >>> + struct kvm_xvmap* (*vmap)(struct kvm_xinterface *intf, >>> + unsigned long gpa, >>> + unsigned long len); >>> >>> >> How would vmap() work with live migration? >> > vmap represents shmem regions, and is a per-guest-instance resource. So > my plan there is that the new and old guest instance would each have the > vmap region instated at the same GPA location (assumption: gpas are > stable across migration), and any state relevant data local to the shmem > (like ring head/tail position) is conveyed in the serialized stream for > the device model. >
You'd have to copy the entire range since you don't know what the guest might put there. I guess it's acceptable for small areas.
>>> + >>> +static inline void >>> +_kvm_xinterface_release(struct kref *kref) >>> +{ >>> + struct kvm_xinterface *intf; >>> + struct module *owner; >>> + >>> + intf = container_of(kref, struct kvm_xinterface, kref); >>> + >>> + owner = intf->owner; >>> + rmb(); >>> >>> >> Why rmb? >> > the intf->ops->release() line may invalidate the intf pointer, so we > want to ensure that the read completes before the release() is called. > > TBH: I'm not 100% its needed, but I was being conservative. >
rmb()s are only needed if an external agent can issue writes, otherwise you'd need one after every statement.
>> >> A simple per-vcpu cache (in struct kvm_vcpu) is likely to give better >> results. >> > per-vcpu will not work well here, unfortunately, since this is an > external interface mechanism. The callers will generally be from a > kthread or some other non-vcpu related context. Even if we could figure > out a vcpu to use as a basis, we would require some kind of > heavier-weight synchronization which would not be as desirable. > > Therefore, I opted to go per-cpu and use the presumably lighterweight > get_cpu/put_cpu() instead. >
This just assumes a low context switch rate.
How about a gfn_to_pfn_cached(..., struct gfn_to_pfn_cache *cache)? Each user can place it in a natural place.
>>> +static unsigned long >>> +xinterface_copy_to(struct kvm_xinterface *intf, unsigned long gpa, >>> + const void *src, unsigned long n) >>> +{ >>> + struct _xinterface *_intf = to_intf(intf); >>> + unsigned long dst; >>> + bool kthread = !current->mm; >>> + >>> + down_read(&_intf->kvm->slots_lock); >>> + >>> + dst = gpa_to_hva(_intf, gpa); >>> + if (!dst) >>> + goto out; >>> + >>> + if (kthread) >>> + use_mm(_intf->mm); >>> + >>> + if (kthread || _intf->mm == current->mm) >>> + n = copy_to_user((void *)dst, src, n); >>> + else >>> + n = _slow_copy_to_user(_intf, dst, src, n); >>> >>> >> Can't you switch the mm temporarily instead of this? >> > Thats actually what I do for the fast-path (use_mm() does a switch_to() > internally). > > The slow-path is only there for completeness for when switching is not > possible (such as if called with an mm already active i.e. > process-context).
Still, why can't you switch temporarily?
> In practice, however, this doesnt happen. Virtually > 100% of the calls in vbus hit the fast-path here, and I suspect most > xinterface clients would find the same conditions as well. >
So you have 100% untested code here.
-- error compiling committee.c: too many arguments to function
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