> -----Original Message-----
> From: Jan Beulich [mailto:[email protected]]
> Sent: 30 August 2018 12:10
> To: xen-devel <[email protected]>
> Cc: Olaf Hering <[email protected]>; Andrew Cooper
> <[email protected]>; Paul Durrant <[email protected]>;
> Wei Liu <[email protected]>
> Subject: [PATCH RFC 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.
>
> Signed-off-by: Jan Beulich <[email protected]>
> ---
> RFC: This clearly wants mirroring to the write path, and perhaps also
> to the fallback code on the RMW path. But I'd like to get a sense
> first on how welcome the general approach is.
At first glance, of course, it appears that there would be a problem when the
second linear_read() after a split returns X86EMUL_RETRY, thus causing the
first one to be re-called when the emulation is re-done. Because the underlying
cache handles the re-call this is not actually a problem though so I think the
approach is ok and should also be fine on the write path.
Paul
>
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -1041,6 +1041,48 @@ static inline int hvmemul_linear_mmio_wr
> pfec, hvmemul_ctxt, translate);
> }
>
> +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, 0);
> +
> + /* 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 __hvmemul_read(
> enum x86_segment seg,
> unsigned long offset,
> @@ -1049,11 +1091,9 @@ 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_vcpu.hvm_io;
> + struct hvm_vcpu_io *vio = ¤t->arch.hvm_vcpu.hvm_io;
> int rc;
>
> if ( is_x86_system_segment(seg) )
> @@ -1073,28 +1113,7 @@ static int __hvmemul_read(
> (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(
>
>
>
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