Re: [PATCH v3 52/75] x86/sev-es: Handle MMIO String Instructions

Borislav Petkov
2020-05-25T09:47:54Z

On Tue, Apr 28, 2020 at 05:17:02PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> Add handling for emulation the MOVS instruction on MMIO regions, as done
> by the memcpy_toio() and memcpy_fromio() functions.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/kernel/sev-es.c | 78 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 78 insertions(+)
>
> diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
> index e3662723ed76..84958a82f8e0 100644
> --- a/arch/x86/kernel/sev-es.c
> +++ b/arch/x86/kernel/sev-es.c
> @@ -552,6 +552,74 @@ static enum es_result vc_handle_mmio_twobyte_ops(struct ghcb *ghcb,
> return ret;
> }
>
> +/*
> + * The MOVS instruction has two memory operands, which raises the
> + * problem that it is not known whether the access to the source or the
> + * destination caused the #VC exception (and hence whether an MMIO read
> + * or write operation needs to be emulated).
> + *
> + * Instead of playing games with walking page-tables and trying to guess
> + * whether the source or destination is an MMIO range, this code splits

s/this code splits/split/

> + * the move into two operations, a read and a write with only one
> + * memory operand. This will cause a nested #VC exception on the MMIO
> + * address which can then be handled.
> + *
> + * This implementation has the benefit that it also supports MOVS where
> + * source _and_ destination are MMIO regions.
> + *
> + * It will slow MOVS on MMIO down a lot, but in SEV-ES guests it is a
> + * rare operation. If it turns out to be a performance problem the split
> + * operations can be moved to memcpy_fromio() and memcpy_toio().
> + */
> +static enum es_result vc_handle_mmio_movs(struct es_em_ctxt *ctxt,
> + unsigned int bytes)
> +{
> + unsigned long ds_base, es_base;
> + unsigned char *src, *dst;
> + unsigned char buffer[8];
> + enum es_result ret;
> + bool rep;
> + int off;
> +
> + ds_base = insn_get_seg_base(ctxt->regs, INAT_SEG_REG_DS);
> + es_base = insn_get_seg_base(ctxt->regs, INAT_SEG_REG_ES);
> +
> + if (ds_base == -1L || es_base == -1L) {
> + ctxt->fi.vector = X86_TRAP_GP;
> + ctxt->fi.error_code = 0;
> + return ES_EXCEPTION;
> + }
> +
> + src = ds_base + (unsigned char *)ctxt->regs->si;
> + dst = es_base + (unsigned char *)ctxt->regs->di;
> +
> + ret = vc_read_mem(ctxt, src, buffer, bytes);
> + if (ret != ES_OK)
> + return ret;
> +
> + ret = vc_write_mem(ctxt, dst, buffer, bytes);
> + if (ret != ES_OK)
> + return ret;
> +
> + if (ctxt->regs->flags & X86_EFLAGS_DF)
> + off = -bytes;
> + else
> + off = bytes;
> +
> + ctxt->regs->si += off;
> + ctxt->regs->di += off;
> +
> + rep = insn_has_rep_prefix(&ctxt->insn);
> +

^ Superfluous newline.

> + if (rep)
> + ctxt->regs->cx -= 1;
> +
> + if (!rep || ctxt->regs->cx == 0)
> + return ES_OK;
> + else
> + return ES_RETRY;
> +}
> +
> static enum es_result vc_handle_mmio(struct ghcb *ghcb,
> struct es_em_ctxt *ctxt)
> {
> @@ -606,6 +674,16 @@ static enum es_result vc_handle_mmio(struct ghcb *ghcb,
> memcpy(reg_data, ghcb->shared_buffer, bytes);
> break;
>
> + /* MOVS instruction */
> + case 0xa4:
> + bytes = 1;
> + /* Fallthrough */

WARNING: Prefer 'fallthrough;' over fallthrough comment
#120: FILE: arch/x86/kernel/sev-es.c:680:
+ /* Fallthrough */

--
Regards/Gruss,
Boris.