Re: [PATCH v3] xen/arm: Convert runstate address during hypcall

[Bertrand Marquis <Bertrand.Marquis@arm.com>]

> On 31 Jul 2020, at 03:18, Stefano Stabellini <sstabellini@kernel.org> wrote:
> 
> On Thu, 30 Jul 2020, Julien Grall wrote:
>> Hi Bertrand,
>> 
>> To avoid extra work on your side, I would recommend to wait a bit before
>> sending a new version. It would be good to at least settle the conversation in
>> v2 regarding the approach taken.
>> 
>> On 30/07/2020 11:24, Bertrand Marquis wrote:
>>> At the moment on Arm, a Linux guest running with KTPI enabled will
>>> cause the following error when a context switch happens in user mode:
>>> (XEN) p2m.c:1890: d1v0: Failed to walk page-table va 0xffffff837ebe0cd0
>>> 
>>> The error is caused by the virtual address for the runstate area
>>> registered by the guest only being accessible when the guest is running
>>> in kernel space when KPTI is enabled.
>>> 
>>> To solve this issue, this patch is doing the translation from virtual
>>> address to physical address during the hypercall and mapping the
>>> required pages using vmap. This is removing the conversion from virtual
>>> to physical address during the context switch which is solving the
>>> problem with KPTI.
>> 
>> To echo what Jan said on the previous version, this is a change in a stable
>> ABI and therefore may break existing guest. FAOD, I agree in principle with
>> the idea. However, we want to explain why breaking the ABI is the *only*
>> viable solution.
>> 
>> From my understanding, it is not possible to fix without an ABI breakage
>> because the hypervisor doesn't know when the guest will switch back from
>> userspace to kernel space. The risk is the information provided by the
>> runstate wouldn't contain accurate information and could affect how the guest
>> handle stolen time.
>> 
>> Additionally there are a few issues with the current interface:
>>   1) It is assuming the virtual address cannot be re-used by the userspace.
>> Thanksfully Linux have a split address space. But this may change with KPTI in
>> place.
>>   2) When update the page-tables, the guest has to go through an invalid
>> mapping. So the translation may fail at any point.
>> 
>> IOW, the existing interface can lead to random memory corruption and
>> inacurracy of the stolen time.
>> 
>>> 
>>> This is done only on arm architecture, the behaviour on x86 is not
>>> modified by this patch and the address conversion is done as before
>>> during each context switch.
>>> 
>>> This is introducing several limitations in comparison to the previous
>>> behaviour (on arm only):
>>> - if the guest is remapping the area at a different physical address Xen
>>> will continue to update the area at the previous physical address. As
>>> the area is in kernel space and usually defined as a global variable this
>>> is something which is believed not to happen. If this is required by a
>>> guest, it will have to call the hypercall with the new area (even if it
>>> is at the same virtual address).
>>> - the area needs to be mapped during the hypercall. For the same reasons
>>> as for the previous case, even if the area is registered for a different
>>> vcpu. It is believed that registering an area using a virtual address
>>> unmapped is not something done.
>> 
>> This is not clear whether the virtual address refer to the current vCPU or the
>> vCPU you register the runstate for. From the past discussion, I think you
>> refer to the former. It would be good to clarify.
>> 
>> Additionally, all the new restrictions should be documented in the public
>> interface. So an OS developper can find the differences between the
>> architectures.
> 
> Just to paraphrase what Julien wrote, it would be good to improve the
> commit message with the points suggested and also write a note in the
> header file about the changes to the interface.

Ok i wil do that.

> 
> 
>> To answer Jan's concern, we certainly don't know all the guest OSes existing,
>> however we also need to balance the benefit for a large majority of the users.
>> 
>> From previous discussion, the current approach was deemed to be acceptable on
>> Arm and, AFAICT, also x86 (see [1]).
>> 
>> TBH, I would rather see the approach to be common. For that, we would an
>> agreement from Andrew and Jan in the approach here. Meanwhile, I think this is
>> the best approach to address the concern from Arm users.
> 
> +1
> 
> 
>>> inline functions in headers could not be used as the architecture
>>> domain.h is included before the global domain.h making it impossible
>>> to use the struct vcpu inside the architecture header.
>>> This should not have any performance impact as the hypercall is only
>>> called once per vcpu usually.
>>> 
>>> Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
>>> 
>>> ---
>>>   Changes in v2
>>>     - use vmap to map the pages during the hypercall.
>>>     - reintroduce initial copy during hypercall.
>>> 
>>>   Changes in v3
>>>     - Fix Coding style
>>>     - Fix vaddr printing on arm32
>>>     - use write_atomic to modify state_entry_time update bit (only
>>>     in guest structure as the bit is not used inside Xen copy)
>>> 
>>> ---
>>>  xen/arch/arm/domain.c        | 161 ++++++++++++++++++++++++++++++-----
>>>  xen/arch/x86/domain.c        |  29 ++++++-
>>>  xen/arch/x86/x86_64/domain.c |   4 +-
>>>  xen/common/domain.c          |  19 ++---
>>>  xen/include/asm-arm/domain.h |   9 ++
>>>  xen/include/asm-x86/domain.h |  16 ++++
>>>  xen/include/xen/domain.h     |   5 ++
>>>  xen/include/xen/sched.h      |  16 +---
>>>  8 files changed, 206 insertions(+), 53 deletions(-)
>>> 
>>> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
>>> index 31169326b2..8b36946017 100644
>>> --- a/xen/arch/arm/domain.c
>>> +++ b/xen/arch/arm/domain.c
>>> @@ -19,6 +19,7 @@
>>>  #include <xen/sched.h>
>>>  #include <xen/softirq.h>
>>>  #include <xen/wait.h>
>>> +#include <xen/vmap.h>
>>>    #include <asm/alternative.h>
>>>  #include <asm/cpuerrata.h>
>>> @@ -275,36 +276,156 @@ static void ctxt_switch_to(struct vcpu *n)
>>>      virt_timer_restore(n);
>>>  }
>>>  -/* Update per-VCPU guest runstate shared memory area (if registered). */
>>> -static void update_runstate_area(struct vcpu *v)
>>> +static void cleanup_runstate_vcpu_locked(struct vcpu *v)
>>>  {
>>> -    void __user *guest_handle = NULL;
>>> +    if ( v->arch.runstate_guest )
>>> +    {
>>> +        vunmap((void *)((unsigned long)v->arch.runstate_guest &
>>> PAGE_MASK));
>>> +
>>> +        put_page(v->arch.runstate_guest_page[0]);
>>> +
>>> +        if ( v->arch.runstate_guest_page[1] )
>>> +            put_page(v->arch.runstate_guest_page[1]);
>>> +
>>> +        v->arch.runstate_guest = NULL;
>>> +    }
>>> +}
>>> +
>>> +void arch_vcpu_cleanup_runstate(struct vcpu *v)
>>> +{
>>> +    spin_lock(&v->arch.runstate_guest_lock);
>>> +
>>> +    cleanup_runstate_vcpu_locked(v);
>>> +
>>> +    spin_unlock(&v->arch.runstate_guest_lock);
>>> +}
>>> +
>>> +static int setup_runstate_vcpu_locked(struct vcpu *v, vaddr_t vaddr)
>>> +{
>>> +    unsigned int offset;
>>> +    mfn_t mfn[2];
>>> +    struct page_info *page;
>>> +    unsigned int numpages;
>>>      struct vcpu_runstate_info runstate;
>>> +    void *p;
>>>  -    if ( guest_handle_is_null(runstate_guest(v)) )
>>> -        return;
>>> +    /* user can pass a NULL address to unregister a previous area */
>>> +    if ( vaddr == 0 )
>>> +        return 0;
>>> +
>>> +    offset = vaddr & ~PAGE_MASK;
>>> +
>>> +    /* provided address must be aligned to a 64bit */
>>> +    if ( offset % alignof(struct vcpu_runstate_info) )
>> 
>> This new restriction wants to be explained in the commit message and public
>> header.
>> 
>>> +    {
>>> +        gprintk(XENLOG_WARNING, "Cannot map runstate pointer at
>>> 0x%"PRIvaddr
>>> +                ": Invalid offset\n", vaddr);
>> 
>> We usually enforce 80 character per lines except for format string. So it is
>> easier to grep them in the code.
>> 
>>> +        return -EINVAL;
>>> +    }
>>> +
>>> +    page = get_page_from_gva(v, vaddr, GV2M_WRITE);
>>> +    if ( !page )
>>> +    {
>>> +        gprintk(XENLOG_WARNING, "Cannot map runstate pointer at
>>> 0x%"PRIvaddr
>>> +                ": Page is not mapped\n", vaddr);
>>> +        return -EINVAL;
>>> +    }
>>> +
>>> +    mfn[0] = page_to_mfn(page);
>>> +    v->arch.runstate_guest_page[0] = page;
>>> +
>>> +    if ( offset > (PAGE_SIZE - sizeof(struct vcpu_runstate_info)) )
>>> +    {
>>> +        /* guest area is crossing pages */
>>> +        page = get_page_from_gva(v, vaddr + PAGE_SIZE, GV2M_WRITE);
>>> +        if ( !page )
>>> +        {
>>> +            put_page(v->arch.runstate_guest_page[0]);
>>> +            gprintk(XENLOG_WARNING,
>>> +                    "Cannot map runstate pointer at 0x%"PRIvaddr
>>> +                    ": 2nd Page is not mapped\n", vaddr);
>>> +            return -EINVAL;
>>> +        }
>>> +        mfn[1] = page_to_mfn(page);
>>> +        v->arch.runstate_guest_page[1] = page;
>>> +        numpages = 2;
>>> +    }
>>> +    else
>>> +    {
>>> +        v->arch.runstate_guest_page[1] = NULL;
>>> +        numpages = 1;
>>> +    }
>>>  -    memcpy(&runstate, &v->runstate, sizeof(runstate));
>>> +    p = vmap(mfn, numpages);
>>> +    if ( !p )
>>> +    {
>>> +        put_page(v->arch.runstate_guest_page[0]);
>>> +        if ( numpages == 2 )
>>> +            put_page(v->arch.runstate_guest_page[1]);
>>>  -    if ( VM_ASSIST(v->domain, runstate_update_flag) )
>>> +        gprintk(XENLOG_WARNING, "Cannot map runstate pointer at
>>> 0x%"PRIvaddr
>>> +                ": vmap error\n", vaddr);
>>> +        return -EINVAL;
>>> +    }
>>> +
>>> +    v->arch.runstate_guest = p + offset;
>>> +
>>> +    if (v == current)
>>> +        memcpy(v->arch.runstate_guest, &v->runstate, sizeof(v->runstate));
>>> +    else
>>>      {
>>> -        guest_handle = &v->runstate_guest.p->state_entry_time + 1;
>>> -        guest_handle--;
>>> -        runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
>>> -        __raw_copy_to_guest(guest_handle,
>>> -                            (void *)(&runstate.state_entry_time + 1) - 1,
>>> 1);
>>> -        smp_wmb();
>>> +        vcpu_runstate_get(v, &runstate);
>>> +        memcpy(v->arch.runstate_guest, &runstate, sizeof(v->runstate));
>>>      }
>>>  -    __copy_to_guest(runstate_guest(v), &runstate, 1);
>>> +    return 0;
>>> +}
>>> +
>>> +int arch_vcpu_setup_runstate(struct vcpu *v,
>>> +                             struct vcpu_register_runstate_memory_area
>>> area)
>>> +{
>>> +    int rc;
>>> +
>>> +    spin_lock(&v->arch.runstate_guest_lock);
>>> +
>>> +    /* cleanup if we are recalled */
>>> +    cleanup_runstate_vcpu_locked(v);
>>> +
>>> +    rc = setup_runstate_vcpu_locked(v, (vaddr_t)area.addr.v);
>>> +
>>> +    spin_unlock(&v->arch.runstate_guest_lock);
>>>  -    if ( guest_handle )
>>> +    return rc;
>>> +}
>>> +
>>> +
>>> +/* Update per-VCPU guest runstate shared memory area (if registered). */
>>> +static void update_runstate_area(struct vcpu *v)
>>> +{
>>> +    spin_lock(&v->arch.runstate_guest_lock);
>>> +
>>> +    if ( v->arch.runstate_guest )
>>>      {
>>> -        runstate.state_entry_time &= ~XEN_RUNSTATE_UPDATE;
>>> -        smp_wmb();
>>> -        __raw_copy_to_guest(guest_handle,
>>> -                            (void *)(&runstate.state_entry_time + 1) - 1,
>>> 1);
>>> +        if ( VM_ASSIST(v->domain, runstate_update_flag) )
>>> +        {
>>> +            v->runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
>>> +            write_atomic(&(v->arch.runstate_guest->state_entry_time),
>>> +                    v->runstate.state_entry_time);
>> 
>> NIT: You want to indent v-> at the same level as the argument from the first
>> line.
>> 
>> Also, I think you are missing a smp_wmb() here.
> 
> I just wanted to add that I reviewed the patch and aside from the
> smp_wmb (and the couple of code style NITs), there is no other issue in
> the patch that I could find. No further comments from my side.
> 
> 
>>> +        }
>>> +
>>> +        memcpy(v->arch.runstate_guest, &v->runstate, sizeof(v->runstate));
>>> +
>>> +        if ( VM_ASSIST(v->domain, runstate_update_flag) )
>>> +        {
>>> +            /* copy must be done before switching the bit */
>>> +            smp_wmb();
>>> +            v->runstate.state_entry_time &= ~XEN_RUNSTATE_UPDATE;
>>> +            write_atomic(&(v->arch.runstate_guest->state_entry_time),
>>> +                    v->runstate.state_entry_time);
>> 
>> Same remark for the indentation.