Re: [RFC PATCH v3 16/36] i386/tdx: Set kvm_readonly_mem_enabled to false for TDX VM

[Isaku Yamahata]

On Thu, Mar 17, 2022 at 09:58:53PM +0800,
Xiaoyao Li <xiaoyao.li@intel.com> wrote:

> TDX only supports readonly for shared memory but not for private memory.
> 
> In the view of QEMU, it has no idea whether a memslot is used by shared
> memory of private. Thus just mark kvm_readonly_mem_enabled to false to
> TDX VM for simplicity.
> 
> Note, pflash has dependency on readonly capability from KVM while TDX
> wants to reuse pflash interface to load TDVF (as OVMF). Excuse TDX VM
> for readonly check in pflash.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> ---
>  hw/i386/pc_sysfw.c    | 2 +-
>  target/i386/kvm/tdx.c | 9 +++++++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/pc_sysfw.c b/hw/i386/pc_sysfw.c
> index c8b17af95353..75b34d02cb4f 100644
> --- a/hw/i386/pc_sysfw.c
> +++ b/hw/i386/pc_sysfw.c
> @@ -245,7 +245,7 @@ void pc_system_firmware_init(PCMachineState *pcms,
>          /* Machine property pflash0 not set, use ROM mode */
>          x86_bios_rom_init(MACHINE(pcms), "bios.bin", rom_memory, false);
>      } else {
> -        if (kvm_enabled() && !kvm_readonly_mem_enabled()) {
> +        if (kvm_enabled() && (!kvm_readonly_mem_enabled() && !is_tdx_vm())) {

Is this called before tdx_kvm_init()?

Thanks,


>              /*
>               * Older KVM cannot execute from device memory. So, flash
>               * memory cannot be used unless the readonly memory kvm
> diff --git a/target/i386/kvm/tdx.c b/target/i386/kvm/tdx.c
> index 94a9c1ea7e9c..1bb8211e74e6 100644
> --- a/target/i386/kvm/tdx.c
> +++ b/target/i386/kvm/tdx.c
> @@ -115,6 +115,15 @@ int tdx_kvm_init(MachineState *ms, Error **errp)
>          get_tdx_capabilities();
>      }
>  
> +    /*
> +     * Set kvm_readonly_mem_allowed to false, because TDX only supports 
> readonly
> +     * memory for shared memory but not for private memory. Besides, whether 
> a
> +     * memslot is private or shared is not determined by QEMU.
> +     *
> +     * Thus, just mark readonly memory not supported for simplicity.
> +     */
> +    kvm_readonly_mem_allowed = false;
> +
>      tdx_guest = tdx;
>  
>      return 0;
> -- 
> 2.27.0
> 
> 

-- 
Isaku Yamahata <isaku.yamahata@gmail.com>