Re: [PATCH v2 2/2] x86/HVM: split page straddling emulated accesses in more cases

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: 03 September 2018 16:45
> To: xen-devel <xen-devel@lists.xenproject.org>
> Cc: Olaf Hering <olaf@aepfle.de>; Andrew Cooper
> <Andrew.Cooper3@citrix.com>; Paul Durrant <Paul.Durrant@citrix.com>
> Subject: [PATCH v2 2/2] x86/HVM: split page straddling emulated accesses in
> more cases
> 
> Assuming consecutive linear addresses map to all RAM or all MMIO is not
> correct. Nor is assuming that a page straddling MMIO access will access
> the same emulating component for both parts of the access. If a guest
> RAM read fails with HVMTRANS_bad_gfn_to_mfn and if the access straddles
> a page boundary, issue accesses separately for both parts.
> 
> The extra call to known_glfn() from hvmemul_write() is just to preserve
> original behavior; we should consider dropping this (also to make sure
> the intended effect of 8cbd4fb0b7 ["x86/hvm: implement hvmemul_write()
> using real mappings"] is achieved in all cases where it can be achieved
> without further rework), or alternatively we perhaps ought to mirror
> this in hvmemul_rmw().
> 
> Note that the correctness of this depends on the MMIO caching used
> elsewhere in the emulation code.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> v2: Also handle hvmemul_{write,rmw}().
> 
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -1041,6 +1041,110 @@ static inline int hvmemul_linear_mmio_wr
>                                        pfec, hvmemul_ctxt, translate);
>  }
> 
> +static bool known_glfn(unsigned long addr, unsigned int bytes, uint32_t
> pfec)
> +{
> +    const struct hvm_vcpu_io *vio = &current->arch.hvm.hvm_io;
> +
> +    if ( pfec & PFEC_write_access )
> +    {
> +        if ( !vio->mmio_access.write_access )
> +            return false;
> +    }
> +    else if ( pfec & PFEC_insn_fetch )
> +    {
> +        if ( !vio->mmio_access.insn_fetch )
> +            return false;
> +    }
> +    else if ( !vio->mmio_access.read_access )
> +            return false;
> +
> +    return (vio->mmio_gla == (addr & PAGE_MASK) &&
> +            (addr & ~PAGE_MASK) + bytes <= PAGE_SIZE);
> +}
> +

Would it be possible to split the introduction of the above function into a separate patch? AFAICT it does not seem to be intrinsically involved with handle page straddling. It was certainly not there in your RFC patch.

  Paul

> +static int linear_read(unsigned long addr, unsigned int bytes, void *p_data,
> +                       uint32_t pfec, struct hvm_emulate_ctxt *hvmemul_ctxt)
> +{
> +    pagefault_info_t pfinfo;
> +    int rc = hvm_copy_from_guest_linear(p_data, addr, bytes, pfec,
> &pfinfo);
> +
> +    switch ( rc )
> +    {
> +        unsigned int offset, part1;
> +
> +    case HVMTRANS_okay:
> +        return X86EMUL_OKAY;
> +
> +    case HVMTRANS_bad_linear_to_gfn:
> +        x86_emul_pagefault(pfinfo.ec, pfinfo.linear, &hvmemul_ctxt->ctxt);
> +        return X86EMUL_EXCEPTION;
> +
> +    case HVMTRANS_bad_gfn_to_mfn:
> +        if ( pfec & PFEC_insn_fetch )
> +            return X86EMUL_UNHANDLEABLE;
> +
> +        offset = addr & ~PAGE_MASK;
> +        if ( offset + bytes <= PAGE_SIZE )
> +            return hvmemul_linear_mmio_read(addr, bytes, p_data, pfec,
> +                                            hvmemul_ctxt,
> +                                            known_glfn(addr, bytes, pfec));
> +
> +        /* Split the access at the page boundary. */
> +        part1 = PAGE_SIZE - offset;
> +        rc = linear_read(addr, part1, p_data, pfec, hvmemul_ctxt);
> +        if ( rc == X86EMUL_OKAY )
> +            rc = linear_read(addr + part1, bytes - part1, p_data + part1,
> +                             pfec, hvmemul_ctxt);
> +        return rc;
> +
> +    case HVMTRANS_gfn_paged_out:
> +    case HVMTRANS_gfn_shared:
> +        return X86EMUL_RETRY;
> +    }
> +
> +    return X86EMUL_UNHANDLEABLE;
> +}
> +
> +static int linear_write(unsigned long addr, unsigned int bytes, void *p_data,
> +                        uint32_t pfec, struct hvm_emulate_ctxt *hvmemul_ctxt)
> +{
> +    pagefault_info_t pfinfo;
> +    int rc = hvm_copy_to_guest_linear(addr, p_data, bytes, pfec, &pfinfo);
> +
> +    switch ( rc )
> +    {
> +        unsigned int offset, part1;
> +
> +    case HVMTRANS_okay:
> +        return X86EMUL_OKAY;
> +
> +    case HVMTRANS_bad_linear_to_gfn:
> +        x86_emul_pagefault(pfinfo.ec, pfinfo.linear, &hvmemul_ctxt->ctxt);
> +        return X86EMUL_EXCEPTION;
> +
> +    case HVMTRANS_bad_gfn_to_mfn:
> +        offset = addr & ~PAGE_MASK;
> +        if ( offset + bytes <= PAGE_SIZE )
> +            return hvmemul_linear_mmio_write(addr, bytes, p_data, pfec,
> +                                             hvmemul_ctxt,
> +                                             known_glfn(addr, bytes, pfec));
> +
> +        /* Split the access at the page boundary. */
> +        part1 = PAGE_SIZE - offset;
> +        rc = linear_write(addr, part1, p_data, pfec, hvmemul_ctxt);
> +        if ( rc == X86EMUL_OKAY )
> +            rc = linear_write(addr + part1, bytes - part1, p_data + part1,
> +                              pfec, hvmemul_ctxt);
> +        return rc;
> +
> +    case HVMTRANS_gfn_paged_out:
> +    case HVMTRANS_gfn_shared:
> +        return X86EMUL_RETRY;
> +    }
> +
> +    return X86EMUL_UNHANDLEABLE;
> +}
> +
>  static int __hvmemul_read(
>      enum x86_segment seg,
>      unsigned long offset,
> @@ -1049,11 +1153,8 @@ static int __hvmemul_read(
>      enum hvm_access_type access_type,
>      struct hvm_emulate_ctxt *hvmemul_ctxt)
>  {
> -    struct vcpu *curr = current;
> -    pagefault_info_t pfinfo;
>      unsigned long addr, reps = 1;
>      uint32_t pfec = PFEC_page_present;
> -    struct hvm_vcpu_io *vio = &curr->arch.hvm.hvm_io;
>      int rc;
> 
>      if ( is_x86_system_segment(seg) )
> @@ -1067,34 +1168,8 @@ static int __hvmemul_read(
>          seg, offset, bytes, &reps, access_type, hvmemul_ctxt, &addr);
>      if ( rc != X86EMUL_OKAY || !bytes )
>          return rc;
> -    if ( ((access_type != hvm_access_insn_fetch
> -           ? vio->mmio_access.read_access
> -           : vio->mmio_access.insn_fetch)) &&
> -         (vio->mmio_gla == (addr & PAGE_MASK)) )
> -        return hvmemul_linear_mmio_read(addr, bytes, p_data, pfec,
> hvmemul_ctxt, 1);
> -
> -    rc = hvm_copy_from_guest_linear(p_data, addr, bytes, pfec, &pfinfo);
> -
> -    switch ( rc )
> -    {
> -    case HVMTRANS_okay:
> -        break;
> -    case HVMTRANS_bad_linear_to_gfn:
> -        x86_emul_pagefault(pfinfo.ec, pfinfo.linear, &hvmemul_ctxt->ctxt);
> -        return X86EMUL_EXCEPTION;
> -    case HVMTRANS_bad_gfn_to_mfn:
> -        if ( access_type == hvm_access_insn_fetch )
> -            return X86EMUL_UNHANDLEABLE;
> -
> -        return hvmemul_linear_mmio_read(addr, bytes, p_data, pfec,
> hvmemul_ctxt, 0);
> -    case HVMTRANS_gfn_paged_out:
> -    case HVMTRANS_gfn_shared:
> -        return X86EMUL_RETRY;
> -    default:
> -        return X86EMUL_UNHANDLEABLE;
> -    }
> 
> -    return X86EMUL_OKAY;
> +    return linear_read(addr, bytes, p_data, pfec, hvmemul_ctxt);
>  }
> 
>  static int hvmemul_read(
> @@ -1171,12 +1246,10 @@ static int hvmemul_write(
>  {
>      struct hvm_emulate_ctxt *hvmemul_ctxt =
>          container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
> -    struct vcpu *curr = current;
>      unsigned long addr, reps = 1;
>      uint32_t pfec = PFEC_page_present | PFEC_write_access;
> -    struct hvm_vcpu_io *vio = &curr->arch.hvm.hvm_io;
>      int rc;
> -    void *mapping;
> +    void *mapping = NULL;
> 
>      if ( is_x86_system_segment(seg) )
>          pfec |= PFEC_implicit;
> @@ -1188,16 +1261,15 @@ static int hvmemul_write(
>      if ( rc != X86EMUL_OKAY || !bytes )
>          return rc;
> 
> -    if ( vio->mmio_access.write_access &&
> -         (vio->mmio_gla == (addr & PAGE_MASK)) )
> -        return hvmemul_linear_mmio_write(addr, bytes, p_data, pfec,
> hvmemul_ctxt, 1);
> -
> -    mapping = hvmemul_map_linear_addr(addr, bytes, pfec, hvmemul_ctxt);
> -    if ( IS_ERR(mapping) )
> -        return ~PTR_ERR(mapping);
> +    if ( !known_glfn(addr, bytes, pfec) )
> +    {
> +        mapping = hvmemul_map_linear_addr(addr, bytes, pfec,
> hvmemul_ctxt);
> +        if ( IS_ERR(mapping) )
> +             return ~PTR_ERR(mapping);
> +    }
> 
>      if ( !mapping )
> -        return hvmemul_linear_mmio_write(addr, bytes, p_data, pfec,
> hvmemul_ctxt, 0);
> +        return linear_write(addr, bytes, p_data, pfec, hvmemul_ctxt);
> 
>      memcpy(mapping, p_data, bytes);
> 
> @@ -1218,7 +1290,6 @@ static int hvmemul_rmw(
>          container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
>      unsigned long addr, reps = 1;
>      uint32_t pfec = PFEC_page_present | PFEC_write_access;
> -    struct hvm_vcpu_io *vio = &current->arch.hvm.hvm_io;
>      int rc;
>      void *mapping;
> 
> @@ -1244,18 +1315,14 @@ static int hvmemul_rmw(
>      else
>      {
>          unsigned long data = 0;
> -        bool known_gpfn = vio->mmio_access.write_access &&
> -                          vio->mmio_gla == (addr & PAGE_MASK);
> 
>          if ( bytes > sizeof(data) )
>              return X86EMUL_UNHANDLEABLE;
> -        rc = hvmemul_linear_mmio_read(addr, bytes, &data, pfec,
> hvmemul_ctxt,
> -                                      known_gpfn);
> +        rc = linear_read(addr, bytes, &data, pfec, hvmemul_ctxt);
>          if ( rc == X86EMUL_OKAY )
>              rc = x86_emul_rmw(&data, bytes, eflags, state, ctxt);
>          if ( rc == X86EMUL_OKAY )
> -            rc = hvmemul_linear_mmio_write(addr, bytes, &data, pfec,
> -                                           hvmemul_ctxt, known_gpfn);
> +            rc = linear_write(addr, bytes, &data, pfec, hvmemul_ctxt);
>      }
> 
>      return rc;
> 
> 


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