Re: [Xen-devel] [PATCH] x86/hvm: make hvmemul_virtual_to_linear()'s reps parameter optional

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <JBeulich@xxxxxxxx>
> Sent: 01 July 2019 13:00
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger 
> Pau Monne
> <roger.pau@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Subject: [PATCH] x86/hvm: make hvmemul_virtual_to_linear()'s reps parameter 
> optional
> 
> A majority of callers wants just a single iteration handled. Allow to
> express this by passing in a NULL pointer, instead of setting up a local
> variable just to hold the "1" to pass in here.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

> ---
> Note that this conflicts with additions/changes made by "x86emul:
> further work". Whatever goes in later will need re-basing.
> 
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -788,14 +788,14 @@ static int hvmemul_virtual_to_linear(
>       enum x86_segment seg,
>       unsigned long offset,
>       unsigned int bytes_per_rep,
> -    unsigned long *reps,
> +    unsigned long *reps_p,
>       enum hvm_access_type access_type,
>       struct hvm_emulate_ctxt *hvmemul_ctxt,
>       unsigned long *linear)
>   {
>       struct segment_register *reg;
>       int okay;
> -    unsigned long max_reps = 4096;
> +    unsigned long reps = 1;
> 
>       if ( seg == x86_seg_none )
>       {
> @@ -803,62 +803,72 @@ static int hvmemul_virtual_to_linear(
>           return X86EMUL_OKAY;
>       }
> 
> -    /*
> -     * If introspection has been enabled for this domain, and we're emulating
> -     * becase a vm_reply asked us to (i.e. not doing regular IO) reps should
> -     * be at most 1, since optimization might otherwise cause a single
> -     * vm_event being triggered for repeated writes to a whole page.
> -     */
> -    if ( unlikely(current->domain->arch.mem_access_emulate_each_rep) &&
> -         current->arch.vm_event->emulate_flags != 0 )
> -       max_reps = 1;
> +    if ( reps_p )
> +    {
> +        unsigned long max_reps = 4096;
> 
> -    /*
> -     * Clip repetitions to avoid overflow when multiplying by @bytes_per_rep.
> -     * The chosen maximum is very conservative but it's what we use in
> -     * hvmemul_linear_to_phys() so there is no point in using a larger value.
> -     */
> -    *reps = min_t(unsigned long, *reps, max_reps);
> +        /*
> +         * If introspection has been enabled for this domain, and we're
> +         * emulating because a vm_reply asked us to (i.e. not doing regular 
> IO)
> +         * reps should be at most 1, since optimization might otherwise 
> cause a
> +         * single vm_event being triggered for repeated writes to a whole 
> page.
> +         */
> +        if ( unlikely(current->domain->arch.mem_access_emulate_each_rep) &&
> +             current->arch.vm_event->emulate_flags != 0 )
> +           max_reps = 1;
> +
> +        /*
> +         * Clip repetitions to avoid overflow when multiplying by
> +         * @bytes_per_rep. The chosen maximum is very conservative but it's
> +         * what we use in hvmemul_linear_to_phys() so there is no point in
> +         * using a larger value.
> +         */
> +        reps = *reps_p = min_t(unsigned long, *reps_p, max_reps);
> +    }
> 
>       reg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
>       if ( IS_ERR(reg) )
>           return -PTR_ERR(reg);
> 
> -    if ( (hvmemul_ctxt->ctxt.regs->eflags & X86_EFLAGS_DF) && (*reps > 1) )
> +    if ( (hvmemul_ctxt->ctxt.regs->eflags & X86_EFLAGS_DF) && (reps > 1) )
>       {
>           /*
>            * x86_emulate() clips the repetition count to ensure we don't wrap
>            * the effective-address index register. Hence this assertion holds.
>            */
> -        ASSERT(offset >= ((*reps - 1) * bytes_per_rep));
> +        ASSERT(offset >= ((reps - 1) * bytes_per_rep));
>           okay = hvm_virtual_to_linear_addr(
> -            seg, reg, offset - (*reps - 1) * bytes_per_rep,
> -            *reps * bytes_per_rep, access_type,
> +            seg, reg, offset - (reps - 1) * bytes_per_rep,
> +            reps * bytes_per_rep, access_type,
>               hvmemul_get_seg_reg(x86_seg_cs, hvmemul_ctxt), linear);
> -        *linear += (*reps - 1) * bytes_per_rep;
> +        *linear += (reps - 1) * bytes_per_rep;
>           if ( hvmemul_ctxt->ctxt.addr_size != 64 )
>               *linear = (uint32_t)*linear;
>       }
>       else
>       {
>           okay = hvm_virtual_to_linear_addr(
> -            seg, reg, offset, *reps * bytes_per_rep, access_type,
> +            seg, reg, offset, reps * bytes_per_rep, access_type,
>               hvmemul_get_seg_reg(x86_seg_cs, hvmemul_ctxt), linear);
>       }
> 
>       if ( okay )
>           return X86EMUL_OKAY;
> 
> -    /* If this is a string operation, emulate each iteration separately. */
> -    if ( *reps != 1 )
> -        return X86EMUL_UNHANDLEABLE;
> +    if ( reps_p )
> +    {
> +        /* If this is a string operation, emulate each iteration separately. 
> */
> +        if ( reps != 1 )
> +            return X86EMUL_UNHANDLEABLE;
> +
> +        *reps_p = 0;
> +    }
> 
>       /*
>        * Leave exception injection to the caller for non-user segments: We
>        * neither know the exact error code to be used, nor can we easily
>        * determine the kind of exception (#GP or #TS) in that case.
>        */
> -    *reps = 0;
>       if ( is_x86_user_segment(seg) )
>           x86_emul_hw_exception((seg == x86_seg_ss)
>                                 ? TRAP_stack_error
> @@ -1201,7 +1211,7 @@ static int __hvmemul_read(
>       enum hvm_access_type access_type,
>       struct hvm_emulate_ctxt *hvmemul_ctxt)
>   {
> -    unsigned long addr, reps = 1;
> +    unsigned long addr;
>       uint32_t pfec = PFEC_page_present;
>       int rc;
> 
> @@ -1213,7 +1223,7 @@ static int __hvmemul_read(
>           pfec |= PFEC_insn_fetch;
> 
>       rc = hvmemul_virtual_to_linear(
> -        seg, offset, bytes, &reps, access_type, hvmemul_ctxt, &addr);
> +        seg, offset, bytes, NULL, access_type, hvmemul_ctxt, &addr);
>       if ( rc != X86EMUL_OKAY || !bytes )
>           return rc;
> 
> @@ -1294,7 +1304,7 @@ static int hvmemul_write(
>   {
>       struct hvm_emulate_ctxt *hvmemul_ctxt =
>           container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
> -    unsigned long addr, reps = 1;
> +    unsigned long addr;
>       uint32_t pfec = PFEC_page_present | PFEC_write_access;
>       int rc;
>       void *mapping = NULL;
> @@ -1305,7 +1315,7 @@ static int hvmemul_write(
>           pfec |= PFEC_user_mode;
> 
>       rc = hvmemul_virtual_to_linear(
> -        seg, offset, bytes, &reps, hvm_access_write, hvmemul_ctxt, &addr);
> +        seg, offset, bytes, NULL, hvm_access_write, hvmemul_ctxt, &addr);
>       if ( rc != X86EMUL_OKAY || !bytes )
>           return rc;
> 
> @@ -1336,13 +1346,13 @@ static int hvmemul_rmw(
>   {
>       struct hvm_emulate_ctxt *hvmemul_ctxt =
>           container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
> -    unsigned long addr, reps = 1;
> +    unsigned long addr;
>       uint32_t pfec = PFEC_page_present | PFEC_write_access;
>       int rc;
>       void *mapping = NULL;
> 
>       rc = hvmemul_virtual_to_linear(
> -        seg, offset, bytes, &reps, hvm_access_write, hvmemul_ctxt, &addr);
> +        seg, offset, bytes, NULL, hvm_access_write, hvmemul_ctxt, &addr);
>       if ( rc != X86EMUL_OKAY || !bytes )
>           return rc;
> 
> @@ -1491,14 +1501,14 @@ static int hvmemul_cmpxchg(
>       struct hvm_emulate_ctxt *hvmemul_ctxt =
>           container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
>       struct vcpu *curr = current;
> -    unsigned long addr, reps = 1;
> +    unsigned long addr;
>       uint32_t pfec = PFEC_page_present | PFEC_write_access;
>       struct hvm_vcpu_io *vio = &curr->arch.hvm.hvm_io;
>       int rc;
>       void *mapping = NULL;
> 
>       rc = hvmemul_virtual_to_linear(
> -        seg, offset, bytes, &reps, hvm_access_write, hvmemul_ctxt, &addr);
> +        seg, offset, bytes, NULL, hvm_access_write, hvmemul_ctxt, &addr);
>       if ( rc != X86EMUL_OKAY )
>           return rc;
> 
> @@ -2292,11 +2302,11 @@ static int hvmemul_invlpg(
>   {
>       struct hvm_emulate_ctxt *hvmemul_ctxt =
>           container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
> -    unsigned long addr, reps = 1;
> +    unsigned long addr;
>       int rc;
> 
>       rc = hvmemul_virtual_to_linear(
> -        seg, offset, 1, &reps, hvm_access_none, hvmemul_ctxt, &addr);
> +        seg, offset, 1, NULL, hvm_access_none, hvmemul_ctxt, &addr);
> 
>       if ( rc == X86EMUL_EXCEPTION )
>       {
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