Re: [Xen-devel] [PATCH v2 3/3] x86/hyperv: L0 assisted TLB flush

[Wei Liu <wl@xen.org>]

On Fri, Feb 14, 2020 at 03:42:17PM +0100, Roger Pau Monné wrote:
[...]
> >  #endif /* __XEN_HYPERV_PRIVIATE_H__  */
> > diff --git a/xen/arch/x86/guest/hyperv/tlb.c b/xen/arch/x86/guest/hyperv/tlb.c
> > index 48f527229e..f68e14f151 100644
> > --- a/xen/arch/x86/guest/hyperv/tlb.c
> > +++ b/xen/arch/x86/guest/hyperv/tlb.c
> > @@ -19,15 +19,185 @@
> >   * Copyright (c) 2020 Microsoft.
> >   */
> >  
> > +#include <xen/cpu.h>
> >  #include <xen/cpumask.h>
> >  #include <xen/errno.h>
> >  
> > +#include <asm/guest/hyperv.h>
> > +#include <asm/guest/hyperv-hcall.h>
> > +#include <asm/guest/hyperv-tlfs.h>
> > +
> >  #include "private.h"
> >  
> > +/*
> > + * It is possible to encode up to 4096 pages using the lower 12 bits
> > + * in an element of gva_list
> > + */
> > +#define HV_TLB_FLUSH_UNIT (4096 * PAGE_SIZE)
> > +
> > +static unsigned int fill_gva_list(uint64_t *gva_list, const void *va,
> > +                                  unsigned int order)
> > +{
> > +    unsigned long start = (unsigned long)va;
> > +    unsigned long end = start + (PAGE_SIZE << order) - 1;
> > +    unsigned int n = 0;
> > +
> > +    do {
> > +        unsigned long remain = end - start;
> > +
> > +        gva_list[n] = start & PAGE_MASK;
> > +
> > +        /*
> > +         * Use lower 12 bits to encode the number of additional pages
> > +         * to flush
> > +         */
> > +        if ( remain >= HV_TLB_FLUSH_UNIT )
> > +        {
> > +            gva_list[n] |= ~PAGE_MASK;
> > +            start += HV_TLB_FLUSH_UNIT;
> > +        }
> > +        else if ( remain )
> 
> remain is always going to be > 0, since the loop condition is end >
> start, and hence this can be a plain else.

Ack.

> 
> > +        {
> > +            gva_list[n] |= (remain - 1) >> PAGE_SHIFT;
> > +            start = end;
> > +        }
> > +
> > +        n++;
> > +    } while ( start < end );
> > +
> > +    return n;
> > +}
> > +
> > +static uint64_t flush_tlb_ex(const cpumask_t *mask, const void *va,
> > +                             unsigned int flags)
> > +{
> > +    struct hv_tlb_flush_ex *flush = this_cpu(hv_input_page);
> > +    int nr_banks;
> > +    unsigned int max_gvas, order = flags & FLUSH_ORDER_MASK;
> > +    uint64_t ret;
> > +
> > +    if ( !flush || local_irq_is_enabled() )
> > +    {
> > +        ASSERT_UNREACHABLE();
> > +        return ~0ULL;
> > +    }
> > +
> > +    if ( !(ms_hyperv.hints & HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED) )
> > +        return ~0ULL;
> > +
> > +    flush->address_space = 0;
> > +    flush->flags = HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES;
> > +    if ( !(flags & FLUSH_TLB_GLOBAL) )
> > +        flush->flags |= HV_FLUSH_NON_GLOBAL_MAPPINGS_ONLY;
> > +
> > +    nr_banks = cpumask_to_vpset(&flush->hv_vp_set, mask);
> > +    if ( nr_banks < 0 )
> > +        return ~0ULL;
> 
> It would be nice to propagate the error code from cpumask_to_vpset,
> but since the function can also return HyperV error codes this doesn't
> make much sense.
> 
> > +
> > +    max_gvas =
> > +        (PAGE_SIZE - sizeof(*flush) - nr_banks *
> > +         sizeof(flush->hv_vp_set.bank_contents[0])) /
> > +        sizeof(uint64_t);       /* gva is represented as uint64_t */
> > +
> > +    /*
> > +     * Flush the entire address space if va is NULL or if there is not
> > +     * enough space for gva_list.
> > +     */
> > +    if ( !va || (PAGE_SIZE << order) / HV_TLB_FLUSH_UNIT > max_gvas )
> > +        ret = hv_do_rep_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE_EX, 0,
> > +                                  nr_banks, virt_to_maddr(flush), 0);
> 
> You could just return hv_do_rep_hypercall(...); here, which will avoid
> the else branch below and the indentation.

Ack.

> 
> > +    else
> > +    {
> > +        uint64_t *gva_list =
> > +            (uint64_t *)flush + sizeof(*flush) / sizeof(uint64_t) + nr_banks;
> > +        unsigned int gvas = fill_gva_list(gva_list, va, order);
> > +
> > +        BUILD_BUG_ON(sizeof(*flush) % sizeof(uint64_t));
> > +
> > +        ret = hv_do_rep_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST_EX,
> > +                                  gvas, nr_banks, virt_to_maddr(flush), 0);
> > +    }
> > +
> > +    return ret;
> > +}
> > +
> >  int hyperv_flush_tlb(const cpumask_t *mask, const void *va,
> >                       unsigned int flags)
> >  {
> > -    return -EOPNOTSUPP;
> > +    unsigned long irq_flags;
> > +    struct hv_tlb_flush *flush = this_cpu(hv_input_page);
> > +    unsigned int max_gvas, order = flags & FLUSH_ORDER_MASK;
> > +    uint64_t ret;
> > +
> > +    ASSERT(flush);
> > +    ASSERT(!cpumask_empty(mask));
> 
> I would also turn this into an if ( ... ) { ASSERT; return -EFOO; }

Ack.

> 
> > +
> > +    local_irq_save(irq_flags);
> > +
> > +    flush->address_space = 0;
> > +    flush->flags = HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES;
> > +    flush->processor_mask = 0;
> > +    if ( !(flags & FLUSH_TLB_GLOBAL) )
> > +        flush->flags |= HV_FLUSH_NON_GLOBAL_MAPPINGS_ONLY;
> > +
> > +    if ( cpumask_equal(mask, &cpu_online_map) )
> > +        flush->flags |= HV_FLUSH_ALL_PROCESSORS;
> > +    else
> > +    {
> > +        unsigned int cpu;
> > +
> > +        /*
> > +         * Normally VP indices are in ascending order and match Xen's
> > +         * idea of CPU ids. Check the last index to see if VP index is
> > +         * >= 64. If so, we can skip setting up parameters for
> > +         * non-applicable hypercalls without looking further.
> > +         */
> > +        if ( hv_vp_index(cpumask_last(mask)) >= 64 )
> > +            goto do_ex_hypercall;
> > +
> > +        for_each_cpu ( cpu, mask )
> > +        {
> > +            uint32_t vpid = hv_vp_index(cpu);
> 
> This should be unsigned int now.

Good catch.

> 
> > +
> > +            if ( vpid > ms_hyperv.max_vp_index )
> > +            {
> > +                local_irq_restore(irq_flags);
> > +                return -ENXIO;
> > +            }
> > +
> > +            if ( vpid >= 64 )
> > +                goto do_ex_hypercall;
> > +
> > +            __set_bit(vpid, &flush->processor_mask);
> > +        }
> 
> Would it make sense to abstract this as cpumask_to_processor_mask,
> since you are adding cpumask_to_vpset below.
> 

There is only one usage so far, so I don't think it is necessary.

> > +    }
> > +
> > +    max_gvas = (PAGE_SIZE - sizeof(*flush)) / sizeof(flush->gva_list[0]);
> 
> You could init this at declaration, and make it const static since the
> value can be calculated at compile time AFAICT. Or create a define
> with it (HV_TLB_FLUSH_MAX_GVAS?). There's no need to store it on the
> stack.

I can introduce a define, but the name you suggested is too generic. If there
is variable sized header, the calculation is going to be different.

> 
> > +
> > +    /*
> > +     * Flush the entire address space if va is NULL or if there is not
> > +     * enough space for gva_list.
> > +     */
> > +    if ( !va || (PAGE_SIZE << order) / HV_TLB_FLUSH_UNIT > max_gvas )
> > +        ret = hv_do_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE,
> > +                              virt_to_maddr(flush), 0);
> > +    else
> > +    {
> > +        unsigned int gvas = fill_gva_list(flush->gva_list, va, order);
> 
> No need for the gvas variable, you can just call fill_gva_list at
> hv_do_rep_hypercall.
> 

Sure.

> > +
> > +        ret = hv_do_rep_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST, gvas, 0,
> > +                                  virt_to_maddr(flush), 0);
> > +    }
> > +
> > +    goto done;
> > +
> > + do_ex_hypercall:
> > +    ret = flush_tlb_ex(mask, va, flags);
> > +
> > + done:
> > +    local_irq_restore(irq_flags);
> > +
> > +    return ret & HV_HYPERCALL_RESULT_MASK ? -ENXIO : 0;
> >  }
> >  
> >  /*
> > diff --git a/xen/arch/x86/guest/hyperv/util.c b/xen/arch/x86/guest/hyperv/util.c
> > new file mode 100644
> > index 0000000000..e092593746
> > --- /dev/null
> > +++ b/xen/arch/x86/guest/hyperv/util.c
> > @@ -0,0 +1,74 @@
> > +/******************************************************************************
> > + * arch/x86/guest/hyperv/util.c
> > + *
> > + * Hyper-V utility functions
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation; either version 2 of the License, or
> > + * (at your option) any later version.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > + * GNU General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU General Public License
> > + * along with this program; If not, see <http://www.gnu.org/licenses/>.
> > + *
> > + * Copyright (c) 2020 Microsoft.
> > + */
> > +
> > +#include <xen/cpu.h>
> > +#include <xen/cpumask.h>
> > +#include <xen/errno.h>
> > +
> > +#include <asm/guest/hyperv.h>
> > +#include <asm/guest/hyperv-tlfs.h>
> > +
> > +#include "private.h"
> > +
> > +int cpumask_to_vpset(struct hv_vpset *vpset,
> > +                     const cpumask_t *mask)
> > +{
> > +    int nr = 1;
> > +    unsigned int cpu, vcpu_bank, vcpu_offset;
> > +    unsigned int max_banks = ms_hyperv.max_vp_index / 64;
> > +
> > +    /* Up to 64 banks can be represented by valid_bank_mask */
> > +    if ( max_banks >= 64 )
> > +        return -E2BIG;
> > +
> > +    /* Clear all banks to avoid flushing unwanted CPUs */
> > +    for ( vcpu_bank = 0; vcpu_bank <= max_banks; vcpu_bank++ )
> 
> I think this is off by one and should be vcpu_bank < max_banks? Or
> else you are clearing one extra bank.

If max_vp_index were the maximum vp index, max_banks would be inclusive
(for example, max_vp_index is 63, 63/64=0).  The code should be correct
in that case.

However, when I did another round of self-review, I discovered that
the value stored in max_vp_index was actually "the maximum number of
virtual processors supported", so it wasn't really the maximum index.

This means I will need to adjust places where max_vp_index is used.

Here, max_banks is not inclusive anymore.

And an earlier place

 +            if ( vpid > ms_hyperv.max_vp_index )
 +            {

also requires fixing.

I will also see about changing max_{l,v}p_index to more sensible names
or add some comments to their definitions.

Wei.

> 
> Thanks, Roger.

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel