Re: [PATCH v2] xen/arm: Convert runstate address during hypcall

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

> On 28 Jul 2020, at 21:04, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> 
> On Tue, 28 Jul 2020, 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.
>> 
>> 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.
>> 
>> 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@xxxxxxx>
>> ---
>>  Changes in v2
>>    - use vmap to map the pages during the hypercall.
>>    - reintroduce initial copy during hypercall.
>> 
>> ---
>> xen/arch/arm/domain.c        | 160 +++++++++++++++++++++++++++++++----
>> xen/arch/x86/domain.c        |  30 ++++++-
>> 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, 207 insertions(+), 52 deletions(-)
>> 
>> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
>> index 31169326b2..c595438bd9 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 +276,157 @@ 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)
>> +{
>> +    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)
>> {
>> -    void __user *guest_handle = NULL;
>> +    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;
>> 
>> -    memcpy(&runstate, &v->runstate, sizeof(runstate));
>> +    offset = vaddr & ~PAGE_MASK;
>> 
>> -    if ( VM_ASSIST(v->domain, runstate_update_flag) )
>> +    /* provided address must be aligned to a 64bit */
>> +    if ( offset % alignof(struct vcpu_runstate_info) )
>>     {
>> -        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();
>> +        gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%lx: "
>> +                "Invalid offset\n", vaddr);
>> +        return -EINVAL;
>> +    }
>> +
>> +    page = get_page_from_gva(v, vaddr, GV2M_WRITE);
>> +    if ( !page )
>> +    {
>> +        gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%lx: "
>> +                "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%lx: "
>> +                    "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;
>> +    }
>> +
>> +    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]);
>> +
>> +        gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%lx: "
>> +                "vmap error\n", vaddr);
>> +        return -EINVAL;
>>     }
>> 
>> -    __copy_to_guest(runstate_guest(v), &runstate, 1);
>> +    v->arch.runstate_guest = p + offset;
>> 
>> -    if ( guest_handle )
>> +    if (v == current)
>>     {
>> -        runstate.state_entry_time &= ~XEN_RUNSTATE_UPDATE;
>> +        memcpy(v->arch.runstate_guest, &v->runstate, sizeof(v->runstate));
>> +    }
>> +    else
>> +    {
>> +        vcpu_runstate_get(v, &runstate);
>> +        memcpy(v->arch.runstate_guest, &runstate, sizeof(v->runstate));
>> +    }
>> +
>> +    return 0;
>> +}
> 
> 
> The arm32 build breaks with:
> 
> domain.c: In function 'setup_runstate_vcpu_locked':
> domain.c:322:9: error: format '%lx' expects argument of type 'long unsigned 
> int', but argument 3 has type 'vaddr_t' [-Werror=format=]
>         gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%lx: "
>         ^
> domain.c:330:9: error: format '%lx' expects argument of type 'long unsigned 
> int', but argument 3 has type 'vaddr_t' [-Werror=format=]
>         gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%lx: "
>         ^
> domain.c:344:13: error: format '%lx' expects argument of type 'long unsigned 
> int', but argument 3 has type 'vaddr_t' [-Werror=format=]
>             gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%lx: "
>             ^
> domain.c:365:9: error: format '%lx' expects argument of type 'long unsigned 
> int', but argument 3 has type 'vaddr_t' [-Werror=format=]
>         gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%lx: "
>         ^
> cc1: all warnings being treated as errors

My bad. I tested x86 and arm64 build but forgot the arm32.
I will fix that.

> 
> 
>> +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);
>> +
>> +    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 )
>> +    {
>> +        if ( VM_ASSIST(v->domain, runstate_update_flag) )
>> +        {
>> +            v->runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
>> +            v->arch.runstate_guest->state_entry_time |= XEN_RUNSTATE_UPDATE;
> 
> Please use write_atomic (as suggested by Julien here:
> https://marc.info/?l=xen-devel&m=159225391619240)

I will do that.

> 
> 
>> +            smp_wmb();
>> +        }
>> +
>> +        memcpy(v->arch.runstate_guest, &v->runstate, sizeof(v->runstate));
>> +
>> +        /* copy must be done before switching the bit */
>>         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;
>> +            v->arch.runstate_guest->state_entry_time &= 
>> ~XEN_RUNSTATE_UPDATE;
> 
> and here too
> 
> The rest looks OK to me.

Thanks for the review.
Regards
Bertrand

> 
> 
>> +        }
>>     }
>> +
>> +    spin_unlock(&v->arch.runstate_guest_lock);
>> }
>> 
>> static void schedule_tail(struct vcpu *prev)