Re: [PATCH] tpm_crb: mark command buffer as dirty on request completion

[Stefan Berger]

On 4/11/22 10:47, Anthony PERARD wrote:
> From: Anthony PERARD <anthony.perard@citrix.com>
>
> At the moment, there doesn't seems to be any way to know that QEMU
> made modification to the command buffer. This is potentially an issue
> on Xen while migrating a guest, as modification to the buffer after
> the migration as started could be ignored and not transfered to the
> destination. >
> Mark the memory region of the command buffer as dirty once a request
> is completed.

Not sure about the CRB...

The TIS at least carries the buffer as part of the state and stores it 
when the interface is saved:

https://github.com/qemu/qemu/blob/v6.2.0/hw/tpm/tpm_tis_isa.c#L56
https://github.com/qemu/qemu/blob/v6.2.0/hw/tpm/tpm_tis_sysbus.c#L56

With the CRB the memory is registered using these functions.

    memory_region_init_io(&s->mmio, OBJECT(s), &tpm_crb_memory_ops, s,
        "tpm-crb-mmio", sizeof(s->regs));
    memory_region_init_ram(&s->cmdmem, OBJECT(s),
        "tpm-crb-cmd", CRB_CTRL_CMD_SIZE, errp);

    memory_region_add_subregion(get_system_memory(),
        TPM_CRB_ADDR_BASE, &s->mmio);
    memory_region_add_subregion(get_system_memory(),
        TPM_CRB_ADDR_BASE + sizeof(s->regs), &s->cmdmem);


The state of the registers is saved using this here:

static const VMStateDescription vmstate_tpm_crb = {
    .name = "tpm-crb",
    .pre_save = tpm_crb_pre_save,
    .fields = (VMStateField[]) {
        VMSTATE_UINT32_ARRAY(regs, CRBState, TPM_CRB_R_MAX),
        VMSTATE_END_OF_LIST(),
    }
};

The buffer with the commands is not part of this. So likely it needs to 
be marked dirty but then I am not sure whether that is going to work if 
the response to a command on the CRB is only received when 
tpm_crb_pre_save() is called.. Maybe this buffer would have to be save 
explicitly in a .save function or also as part of vmstate_tpm_crb... ?


  Stefan




> Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
> ---
>
> I have only read code to find out whether the tpm-crb device was fine
> with regards to migration, and I don't think there's anything that
> could mark the memory region as dirty once a request is completed.
>
> There is one call to memory_region_get_ram_ptr(), but nothing seems to
> be done with the pointer is regards to ram migration. Am I wrong?
>
> Thanks.
> ---
>   hw/tpm/tpm_crb.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/hw/tpm/tpm_crb.c b/hw/tpm/tpm_crb.c
> index aa9c00aad3..67db594c48 100644
> --- a/hw/tpm/tpm_crb.c
> +++ b/hw/tpm/tpm_crb.c
> @@ -197,6 +197,7 @@ static void tpm_crb_request_completed(TPMIf *ti, int ret)
>           ARRAY_FIELD_DP32(s->regs, CRB_CTRL_STS,
>                            tpmSts, 1); /* fatal error */
>       }
> +    memory_region_set_dirty(&s->cmdmem, 0, CRB_CTRL_CMD_SIZE);
>   }
>
>   static enum TPMVersion tpm_crb_get_version(TPMIf *ti)