Re: [PATCH v3 48/49] hw/i386/sev: Use guest_memfd for legacy ROMs

[Xiaoyao Li]

On 3/21/2024 2:12 AM, Isaku Yamahata wrote:
> On Wed, Mar 20, 2024 at 03:39:44AM -0500,
> Michael Roth <michael.roth@amd.com> wrote:
>
> > TODO: make this SNP-specific if TDX disables legacy ROMs in general
>
> TDX disables pc.rom, not disable isa-bios. IIRC, TDX doesn't need pc pflash.

Not TDX doesn't need pc pflash, but TDX cannot support pflash.

Can SNP support the behavior of pflash? That what's got changed will be 
synced back to OVMF file?

> Xiaoyao can chime in.
>
> Thanks,
>
> > Current SNP guest kernels will attempt to access these regions with
> > with C-bit set, so guest_memfd is needed to handle that. Otherwise,
> > kvm_convert_memory() will fail when the guest kernel tries to access it
> > and QEMU attempts to call KVM_SET_MEMORY_ATTRIBUTES to set these ranges
> > to private.
> >
> > Whether guests should actually try to access ROM regions in this way (or
> > need to deal with legacy ROM regions at all), is a separate issue to be
> > addressed on kernel side, but current SNP guest kernels will exhibit
> > this behavior and so this handling is needed to allow QEMU to continue
> > running existing SNP guest kernels.
> >
> > Signed-off-by: Michael Roth <michael.roth@amd.com>
> > ---
> >   hw/i386/pc.c       | 13 +++++++++----
> >   hw/i386/pc_sysfw.c | 13 ++++++++++---
> >   2 files changed, 19 insertions(+), 7 deletions(-)
> >
> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > index feb7a93083..5feaeb43ee 100644
> > --- a/hw/i386/pc.c
> > +++ b/hw/i386/pc.c
> > @@ -1011,10 +1011,15 @@ void pc_memory_init(PCMachineState *pcms,
> >       pc_system_firmware_init(pcms, rom_memory);
> >   
> >       option_rom_mr = g_malloc(sizeof(*option_rom_mr));
> > -    memory_region_init_ram(option_rom_mr, NULL, "pc.rom", PC_ROM_SIZE,
> > -                           &error_fatal);
> > -    if (pcmc->pci_enabled) {
> > -        memory_region_set_readonly(option_rom_mr, true);
> > +    if (machine_require_guest_memfd(machine)) {
> > +        memory_region_init_ram_guest_memfd(option_rom_mr, NULL, "pc.rom",
> > +                                           PC_ROM_SIZE, &error_fatal);
> > +    } else {
> > +        memory_region_init_ram(option_rom_mr, NULL, "pc.rom", PC_ROM_SIZE,
> > +                               &error_fatal);
> > +        if (pcmc->pci_enabled) {
> > +            memory_region_set_readonly(option_rom_mr, true);
> > +        }
> >       }
> >       memory_region_add_subregion_overlap(rom_memory,
> >                                           PC_ROM_MIN_VGA,
> > diff --git a/hw/i386/pc_sysfw.c b/hw/i386/pc_sysfw.c
> > index 9dbb3f7337..850f86edd4 100644
> > --- a/hw/i386/pc_sysfw.c
> > +++ b/hw/i386/pc_sysfw.c
> > @@ -54,8 +54,13 @@ static void pc_isa_bios_init(MemoryRegion *rom_memory,
> >       /* map the last 128KB of the BIOS in ISA space */
> >       isa_bios_size = MIN(flash_size, 128 * KiB);
> >       isa_bios = g_malloc(sizeof(*isa_bios));
> > -    memory_region_init_ram(isa_bios, NULL, "isa-bios", isa_bios_size,
> > -                           &error_fatal);
> > +    if (machine_require_guest_memfd(current_machine)) {
> > +        memory_region_init_ram_guest_memfd(isa_bios, NULL, "isa-bios",
> > +                                           isa_bios_size, &error_fatal);
> > +    } else {
> > +        memory_region_init_ram(isa_bios, NULL, "isa-bios", isa_bios_size,
> > +                               &error_fatal);
> > +    }
> >       memory_region_add_subregion_overlap(rom_memory,
> >                                           0x100000 - isa_bios_size,
> >                                           isa_bios,
> > @@ -68,7 +73,9 @@ static void pc_isa_bios_init(MemoryRegion *rom_memory,
> >              ((uint8_t*)flash_ptr) + (flash_size - isa_bios_size),
> >              isa_bios_size);
> >   
> > -    memory_region_set_readonly(isa_bios, true);
> > +    if (!machine_require_guest_memfd(current_machine)) {
> > +        memory_region_set_readonly(isa_bios, true);
> > +    }
> >   }
> >   
> >   static PFlashCFI01 *pc_pflash_create(PCMachineState *pcms,
> > --
> > 2.25.1