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Merged
Lencerf merged 2 commits into from
Mar 8, 2026
Merged

fix(mmio): return all-ones for reading unmapped addresses #422

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e0978f6
Revert "fix(mmio): handle unaligned and partial read/write"
Lencerf Mar 7, 2026
2cde20e
fix(mmio): return all-ones for reading unmapped addresses
Lencerf Mar 7, 2026
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3 changes: 2 additions & 1 deletion alioth/src/hv/kvm/vcpu/vmexit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,10 +22,11 @@ impl KvmVcpu {
#[cfg(target_endian = "little")]
pub(super) fn handle_mmio(&mut self) -> Result<VmExit, Error> {
let kvm_mmio = unsafe { &self.kvm_run.exit.mmio };
let data = u64::from_ne_bytes(kvm_mmio.data) & u64::MAX >> (64 - (kvm_mmio.len << 3));
let exit = VmExit::Mmio {
addr: kvm_mmio.phys_addr,
write: if kvm_mmio.is_write > 0 {
Some(u64::from_ne_bytes(kvm_mmio.data))
Some(data)
} else {
None
},
Expand Down
60 changes: 6 additions & 54 deletions alioth/src/mem/emulated.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,13 +12,11 @@
// See the License for the specific language governing permissions and
// limitations under the License.

use std::cmp::min;
use std::fmt::Debug;
use std::sync::Arc;

use crate::mem::addressable::{Addressable, SlotBackend};
use crate::mem::{Memory, Result};
use crate::utils::truncate_u64;

#[cfg(not(test))]
pub trait ChangeLayout: Debug + Send + Sync + 'static {
Expand Down Expand Up @@ -158,63 +156,17 @@ where
}

pub fn read(&self, addr: u64, size: u8) -> Result<u64> {
let mut count = 0;
let mut val = 0;
let size = size as u64;
while count < size {
let base_addr = addr + count;
let Some((start, dev)) = self.inner.search_next(base_addr) else {
break;
};
count += start.saturating_sub(base_addr);
let offset = base_addr.saturating_sub(start);
let mut read_size = min(Mmio::size(dev) - offset, size.saturating_sub(count));
if read_size == 0 {
break;
}
read_size = min(read_size, 1 << read_size.trailing_zeros());
if offset > 0 {
read_size = min(read_size, 1 << offset.trailing_zeros());
}
val |= truncate_u64(dev.read(offset, read_size as u8)?, read_size) << (count << 3);
count += read_size;
match self.inner.search(addr) {
Some((start, dev)) => dev.read(addr - start, size),
None => Ok(u64::MAX),
}
Ok(val)
}

pub fn write(&self, addr: u64, size: u8, val: u64) -> Result<Action> {
let mut count = 0;
let size = size as u64;
let mut action = Action::None;
while count < size {
let base_addr = addr + count;
let Some((start, dev)) = self.inner.search_next(base_addr) else {
break;
};
count += start.saturating_sub(base_addr);
let offset = base_addr.saturating_sub(start);
let mut write_size = min(Mmio::size(dev) - offset, size.saturating_sub(count));
if write_size == 0 {
break;
}
write_size = min(write_size, 1 << write_size.trailing_zeros());
if offset > 0 {
write_size = min(write_size, 1 << offset.trailing_zeros());
}
let write_val = truncate_u64(val >> (count << 3), write_size);
let r = dev.write(offset, write_size as u8, write_val)?;
if matches!(action, Action::None) {
action = r
} else {
// TODO: handle multiple side effects caused by a single write
log::error!(
"Write {write_val:#x} to {:#x}: dropped: {action:#x?}",
start + offset
);
}
count += write_size;
match self.inner.search(addr) {
Some((start, dev)) => dev.write(addr - start, size, val),
None => Ok(Action::None),
}
Ok(action)
}
}

Expand Down
73 changes: 35 additions & 38 deletions alioth/src/mem/emulated_test.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -31,18 +31,19 @@ impl Mmio for TestRange {
self.size
}

fn read(&self, offset: u64, _size: u8) -> Result<u64> {
fn read(&self, offset: u64, size: u8) -> Result<u64> {
let val = *self.val.lock() >> (offset << 3);
Ok(val)
let mask = u64::MAX >> (64 - (size << 3));
Ok(val & mask)
}

fn write(&self, offset: u64, size: u8, val: u64) -> Result<Action> {
assert_eq!(size.count_ones(), 1);
assert!(offset.trailing_zeros() >= size.trailing_zeros());
let v = &mut *self.val.lock();
let shift = offset << 3;
*v &= !(((1 << (size << 3)) - 1) << shift);
*v |= val << shift;
let mask = u64::MAX >> (64 - (size << 3));
*v &= !(mask << shift);
*v |= (val & mask) << shift;
Ok(Action::None)
}
}
Expand Down Expand Up @@ -74,53 +75,49 @@ fn fixture_mmio_bus() -> MmioBus {

#[rstest]
#[case(0x0, 1, 0x01)]
#[case(0x0, 2, 0x2301)]
#[case(0x0, 3, 0x672301)]
#[case(0x0, 4, 0x45672301)]
#[case(0x0, 8, 0x89ab_cdef_4567_2301)]
#[case(0x0, 2, 0xff01)]
#[case(0x0, 3, 0xff_ff01)]
#[case(0x0, 4, 0xffff_ff01)]
#[case(0x0, 8, 0xffff_ffff_ffff_ff01)]
#[case(0x1, 1, 0x23)]
#[case(0x1, 2, 0x6723)]
#[case(0x1, 3, 0x45_6723)]
#[case(0x1, 4, 0xef45_6723)]
#[case(0x1, 8, 0x89_abcd_ef45_6723)]
#[case(0x4, 8, 0x89ab_cdef)]
#[case(0x1, 2, 0xff23)]
#[case(0x1, 3, 0xff_ff23)]
#[case(0x1, 4, 0xffff_ff23)]
#[case(0x1, 8, 0xffff_ffff_ffff_ff23)]
#[case(0x4, 8, 0xffff_ffff_89ab_cdef)]
#[case(0x6, 1, 0xab)]
#[case(0x6, 2, 0x89ab)]
#[case(0x6, 4, 0x89ab)]
#[case(0x8, 1, 0x0)]
#[case(0x8, 4, 0x0)]
#[case(0x8, 5, 0x34_0000_0000)]
#[case(0x8, 8, 0xabcd_1234_0000_0000)]
#[case(0xa, 8, 0x0000_abcd_1234_0000)]
#[case(0x6, 4, 0xffff_89ab)]
#[case(0x8, 1, u64::MAX)]
#[case(0xa, 8, u64::MAX)]
#[case(0xe, 2, 0xabcd)]
fn test_mmio_bus_read(
fixture_mmio_bus: MmioBus,
#[case] addr: u64,
#[case] size: u8,
#[case] val: u64,
) {
assert_eq!(fixture_mmio_bus.read(addr, size).unwrap(), val)
assert_eq!(
fixture_mmio_bus.read(addr, size).unwrap(),
val,
"Read from addr {addr:#x} with size {size} failed"
)
}

#[rstest]
#[case(0x0, 1, 0x3210, 0x89ab_cdef_4567_2310)]
#[case(0x0, 2, 0x3210, 0x89ab_cdef_4567_3210)]
#[case(0x0, 3, 0x763201, 0x89ab_cdef_4576_3201)]
#[case(0x0, 4, 0x10, 0x89ab_cdef_0000_0010)]
#[case(0x0, 1, 0x3210, 0xffff_ffff_ffff_ff10)]
#[case(0x0, 2, 0x3210, 0xffff_ffff_ffff_3210)]
#[case(0x0, 4, 0x10, 0xffff_ffff_0000_0010)]
#[case(0x0, 8, 0x10, 0x10)]
#[case(0x1, 1, 0x32, 0x89_abcd_ef45_6732)]
#[case(0x1, 2, 0x7632, 0x89_abcd_ef45_7632)]
#[case(0x1, 3, 0x1254_7632, 0x89_abcd_ef54_7632)]
#[case(0x1, 4, 0xfe54_7632, 0x89_abcd_fe54_7632)]
#[case(0x1, 1, 0x32, 0xffff_ffff_ffff_ff32)]
#[case(0x1, 2, 0x7632, 0xffff_ffff_ffff_7632)]
#[case(0x1, 4, 0xfe54_7632, 0xffff_ffff_fe54_7632)]
#[case(0x1, 8, 0x0, 0x0)]
#[case(0x4, 8, 0x1234_89ab_cdef, 0x89ab_cdef)]
#[case(0x6, 1, 0xba, 0x1234_0000_0000_89ba)]
#[case(0x6, 2, 0x98ba, 0x1234_0000_0000_98ba)]
#[case(0x6, 4, 0xff_ab98, 0x1234_0000_0000_ab98)]
#[case(0x8, 1, 0xff, 0xabcd_1234_0000_0000)]
#[case(0x8, 4, 0xffff, 0xabcd_1234_0000_0000)]
#[case(0x8, 5, 0xfe_1234_0000, 0xabcd_12fe_0000_0000)]
#[case(0x8, 8, 0x5678_cdfe_1234_0000, 0x5678_cdfe_0000_0000)]
#[case(0xa, 8, 0xcdfe_1234_abcd, 0xcdfe_1234_0000)]
#[case(0x4, 8, 0x1234_89ab_cdef, 0x1234_89ab_cdef)]
#[case(0x6, 1, 0xba, 0xffff_ffff_89ba)]
#[case(0x6, 2, 0x98ba, 0xffff_ffff_98ba)]
#[case(0x6, 4, 0xcd_ab98, 0xffff_00cd_ab98)]
#[case(0x8, 1, 0xff, u64::MAX)]
fn test_mmio_bus_write(
fixture_mmio_bus: MmioBus,
#[case] addr: u64,
Expand Down